diff --git a/README.md b/README.md
index 9528d7a..dd055f8 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # @vimazing/vim-sudoku
 
-Lightweight, typed **React hooks** and utilities for building interactive sudoku games with VIM-inspired controls.
+Lightweight, typed **React hooks** and utilities for building interactive sudoku games with **VIM-style modal editing**.
 
 Part of the [VIMazing](https://github.com/andrepadez/vimazing-vimaze) project.
 
@@ -10,8 +10,9 @@ Part of the [VIMazing](https://github.com/andrepadez/vimazing-vimaze) project.
 - [Features](#features)
 - [Installation](#installation)
 - [Quick Start](#quick-start)
+- [VIM Modal Editing](#vim-modal-editing)
 - [Core Hooks](#core-hooks)
-- [Utilities](#utilities)
+- [Game Mechanics](#game-mechanics)
 - [Example App](#example-app)
 - [Build & Release](#build--release)
 - [License](#license)
@@ -19,11 +20,15 @@ Part of the [VIMazing](https://github.com/andrepadez/vimazing-vimaze) project.
 ---
 
 ## Features
-- **Drop-in hooks** for sudoku game logic, player control, scoring, and key logging.
-- **Typed API** with generated declaration files for editor IntelliSense.
-- **VIM-style navigation** (`h`, `j`, `k`, `l`, counts, `i` to start, etc.).
-- **Composable architecture** – bring your own rendering and platform-specific bindings.
-- **Tree-shakeable exports** to keep bundles lean.
+- 🎮 **VIM modal editing** – Navigate in normal mode, edit cells in insert mode (R/C to enter)
+- ⌨️ **Full VIM motions** – `h`/`j`/`k`/`l`, counted moves, `^`/`$`/`0` for row navigation
+- 🎯 **Smart cell locking** – Pre-filled clues are protected with visual feedback
+- ⏱️ **Time & keystroke limits** – Configurable game-over conditions
+- 📊 **Built-in scoring** – Time-based scoring with keystroke tracking
+- 🎨 **Tokyo Night theme** – Beautiful dark theme with 3x3 box separators
+- 📦 **Typed API** – Full TypeScript support with generated declarations
+- 🪝 **Composable hooks** – Separate concerns for game state, rendering, input, and scoring
+- 🌲 **Tree-shakeable exports** – Import only what you need
 
 ---
 
@@ -46,65 +51,187 @@ bun add @vimazing/vim-sudoku
 ## Quick Start
 
 ```tsx
-import { useEffect } from "react";
 import { useGame } from "@vimazing/vim-sudoku";
+import "@vimazing/vim-sudoku/sudoku.css";
 
 export function SudokuGame() {
-  const { containerRef, gameStatus, startGame } = useGame();
-
-  useEffect(() => {
-    startGame();
-  }, [startGame]);
+  const { containerRef, initGame, gameStatus, scoreManager } = useGame();
 
   return (
-    <section className="mx-auto w-fit space-y-4">
-      <h1 className="text-2xl font-bold">VIMazing Sudoku</h1>
-      <div ref={containerRef} className="relative" />
-      {gameStatus === "game-won" && <p>🎉 You solved it!</p>}
-    </section>
+    <div>
+      <h1>VIMazing Sudoku</h1>
+      <button onClick={initGame}>New Puzzle</button>
+      
+      <div className="relative w-fit">
+        <div ref={containerRef} />
+      </div>
+      
+      {gameStatus === "game-won" && (
+        <p>🎉 Score: {scoreManager.finalScore?.toLocaleString()}</p>
+      )}
+    </div>
   );
 }
 ```
 
-Default controls:
+---
 
-| Key                 | Action          |
-| ------------------- | --------------- |
-| `i`                 | Start game      |
-| `h` / `j` / `k` / `l` | Move cursor   |
-| `<number>`          | Enter number    |
-| `x` / `d`           | Clear cell      |
-| `q`                 | Quit game       |
-| `Esc`               | Cancel / pause  |
+## VIM Modal Editing
+
+VIMazing Sudoku follows VIM's modal editing philosophy with strict insert/replace semantics:
+
+### Navigation Mode (default)
+
+#### Movement
+
+| Key | Action | Notes |
+| --- | ------ | ----- |
+| `h` / `j` / `k` / `l` | Move cursor left/down/up/right | Core VIM navigation |
+| `<count>h/j/k/l` | Move with count | e.g., `3j` = down 3, `5l` = right 5 |
+| `0` / `^` | Jump to start of row | Move to column 0 |
+| `$` | Jump to end of row | Move to column 8 |
+| `.` | Repeat last motion | Repeat with optional new count |
+
+#### Editing
+
+| Key | Action | Valid On |
+| --- | ------ | -------- |
+| `I` | Enter insert mode | ✅ Empty cells only |
+| `R` | Enter replace mode | ✅ User-filled cells only |
+| `D` / `X` | Delete cell value | ✅ User-filled cells only |
+
+#### Game Control
+
+| Key | Action |
+| --- | ------ |
+| `Q` | Quit game |
+
+### Edit Mode (I/R to enter)
+
+| Key | Action |
+| --- | ------ |
+| `1-9` | Fill cell with number |
+| `x` / `Backspace` | Clear cell |
+| `Esc` / `Enter` | Exit edit mode back to navigation |
+
+---
+
+## Cell State Rules
+
+Understanding cell states is crucial for VIM-style editing:
+
+| Cell State | `I` (Insert) | `R` (Replace) | `D`/`X` (Delete) | Visual |
+| ---------- | ------------ | ------------- | ---------------- | ------ |
+| **Empty** (`.`) | ✅ Enter edit | 🔴 Flash (use I) | ➖ No effect | Normal |
+| **User-filled** | 🔴 Flash (use R) | ✅ Enter edit | ✅ Delete | Normal |
+| **Pre-filled clue** | 🔴 Flash (locked) | 🔴 Flash (locked) | 🔴 Flash (locked) | Darker text |
+
+### Visual Feedback
+
+- 🔵 **Blue glow** – Navigation mode (current cell)
+- 🟡 **Yellow pulsing glow** – Edit mode (typing active)
+- 🔴 **Red flash** – Invalid action (wrong mode, locked cell, or out of bounds)
+
+### Examples
+
+```
+Empty cell:     Press I → type 5 → Esc
+Replace value:  Press R → type 7 → Esc  
+Delete value:   Press D (instant)
+Move 3 down:    Press 3j
+Jump to end:    Press $
+```
 
 ---
 
 ## Core Hooks
 
-| Hook | Description |
-| ---- | ----------- |
-| `useGame(platformHook?)` | One-stop hook that wires sudoku generation, rendering, player state, scoring, and keyboard bindings. Returns refs, status helpers, and managers you can compose with your UI. |
-| `useKeyBindings()` | Provides VIM-style key bindings, including repeat counts and movement helpers. |
-| `useScore()` | Tracks timers, moves, and final score calculation. |
+### `useGame(platformHook?)`
 
-Each hook is exported individually, so you can cherry-pick only what you need:
+Main hook that orchestrates the entire sudoku game. Returns:
+
+```ts
+{
+  // Core
+  containerRef: RefObject<HTMLDivElement>,
+  gameStatus: GameStatus,  // "waiting" | "started" | "game-won" | "game-over"
+  startGame: () => void,
+  stopGame: () => void,
+  initGame: () => void,    // Reset board + start
+  
+  // Cursor
+  cursor: { r: number; c: number },
+  moveCursor: (dr, dc, steps?) => void,
+  
+  // Board
+  board: string,           // 81-character string
+  initialBoard: string,    // Original clues
+  setDigit: (digit) => void,
+  erase: () => void,
+  reset: () => void,
+  solveBoard: () => void,
+  boardToString: (human?) => string,  // Format board for display
+  
+  // Scoring
+  scoreManager: {
+    timeValue: number,     // Time in milliseconds
+    finalScore: number | null,
+    timeLimit: number,     // 60000ms (60 seconds)
+    keystrokeLimit: number, // 100 keystrokes
+  },
+  
+  // Key logging
+  keyLog: KeyLogEntry[],
+  editMode: "navigation" | "edit",
+}
+```
+
+### Individual Hooks
+
+Export individual hooks for granular control:
 
 ```ts
 import { useScore } from "@vimazing/vim-sudoku/useScore";
+import { useBoardState } from "@vimazing/vim-sudoku/useGame/useBoardState";
 ```
 
 ---
 
-## Utilities
+## Game Mechanics
 
-Besides the hooks, the library exports:
+### Scoring
 
-- `SudokuGenerator` – procedural sudoku puzzle generation class.
-- `SudokuRenderer` – canvas renderer tailored for sudoku grids.
-- `types` – shared TypeScript types such as `GameStatus`, `SudokuCell`, and `KeyLogEntry`.
+Time-based scoring formula:
+```
+score = max(0, 100000 - timeInSeconds × 10)
+```
+
+### Game Over Conditions
+
+- ⏱️ **Time limit**: 60 seconds
+- ⌨️ **Keystroke limit**: 100 keystrokes
+
+Customize in `src/useScore/useScore.ts`:
+```ts
+const TIME_LIMIT_MS = 60000;
+const KEYSTROKE_LIMIT = 100;
+```
+
+### Win Detection
+
+Currently manual (solving via UI or `solveBoard()`). Automatic win detection coming soon!
+
+### Board Format
 
 ```ts
-import { SudokuGenerator, GameStatus } from "@vimazing/vim-sudoku";
+// Computer format (compact)
+boardToString()  // "628.1...535928671..."
+
+// Human format (readable)
+boardToString(true)
+// 6 2 8 | 1 . . | . 5 .
+// 5 3 9 | 2 4 8 | 6 7 1
+// ...
 ```
 
 ---
diff --git a/example/src/useKeyBindings.ts b/example/src/useKeyBindings.ts
index 4ec3a28..9290af4 100644
--- a/example/src/useKeyBindings.ts
+++ b/example/src/useKeyBindings.ts
@@ -1,21 +1,22 @@
 import { useEffect } from "react";
-import type { UseGameType } from "../../src/useGame/useGame";
+import type { UseGameType } from "../../src/";
 
 export const useKeyBindings = (gameManager: UseGameType) => {
   const { gameStatus, initGame, stopGame, clearLog, resetCount } = gameManager;
 
   useEffect(() => {
     const handler = (e: KeyboardEvent) => {
+      console.log(e.key);
       // start game
-      if (e.key === "i" || e.key === "I") {
+      if (e.code === "Space") {
         if (["waiting", "game-over", "game-won"].includes(gameStatus)) {
           clearLog();
           resetCount();
           if (gameStatus === "game-over" || gameStatus === "game-won") {
             stopGame();
-            setTimeout(() => initGame(true), 0);
+            setTimeout(() => initGame(), 0);
           } else {
-            initGame(true);
+            initGame();
           }
           return;
         }
diff --git a/example/src/GameOverlay.tsx b/src/GameOverlay.tsx
similarity index 100%
rename from example/src/GameOverlay.tsx
rename to src/GameOverlay.tsx
diff --git a/example/src/MobileGameOverlay.tsx b/src/MobileGameOverlay.tsx
similarity index 100%
rename from example/src/MobileGameOverlay.tsx
rename to src/MobileGameOverlay.tsx
diff --git a/src/sudoku.css b/src/sudoku.css
index 11131c0..50469ff 100644
--- a/src/sudoku.css
+++ b/src/sudoku.css
@@ -1,20 +1,27 @@
 /* =========================================================
    🔧 Controls (tweak these)
    ========================================================= */
+/* :root { */
+/*   --cell-size: 32px; */
+/*   --cell-font-size: 16px; */
+/*   --grid-gap: 2px; */
+/*   --box-separator: 3px; */
+/* } */
+
 :root {
-  --cell-size: 32px;
-  --cell-font-size: 16px;
-  --grid-gap: 2px;
-  --box-separator: 3px;
+  --cell-size: 48px;
+  --cell-font-size: 32px;
+  --grid-gap: 4px;
+  --box-separator: 6px;
 }
 
 
-:root {
-  --cell-size: 96px;
-  --cell-font-size: 48px;
-  --grid-gap: 6px;
-  --box-separator: 9px;
-}
+/* :root { */
+/*   --cell-size: 96px; */
+/*   --cell-font-size: 48px; */
+/*   --grid-gap: 6px; */
+/*   --box-separator: 9px; */
+/* } */
 
 /* =========================================================
    Container & Grid Base
@@ -27,7 +34,7 @@
 
 #sudoku {
   position: relative;
-  background-color: #1e2030; /* Tokyo Night card background */
+  background-color: var(--card, #1e2030);
   border-radius: 0.5rem;
   box-shadow: 0 0 10px rgba(27, 29, 43, 0.6), inset 0 0 6px rgba(130, 170, 255, 0.05);
   padding: 0.5rem;
@@ -64,8 +71,8 @@
   border-radius: 4px;
   font-family: "Fira Code", monospace;
   font-size: var(--cell-font-size);
-  background: #2f334d; /* Tokyo Night muted */
-  color: #c8d3f5; /* Tokyo Night foreground */
+  background: var(--muted, #2f334d);
+  color: var(--foreground, #c8d3f5);
   transition: all 0.15s ease;
   box-shadow: inset 0 0 3px rgba(27, 29, 43, 0.4);
 }
@@ -77,35 +84,35 @@
 #sudoku .sudoku-cell[data-c="2"],
 #sudoku .sudoku-cell[data-c="5"],
 #sudoku .sudoku-cell[data-c="8"] {
-  border-right: 2px solid rgba(130, 170, 255, 0.3);
+  border-right: 2px solid color-mix(in srgb, var(--primary, #82aaff) 30%, transparent);
 }
 
 /* Left borders (start of each 3x3 box horizontally) */
 #sudoku .sudoku-cell[data-c="0"],
 #sudoku .sudoku-cell[data-c="3"],
 #sudoku .sudoku-cell[data-c="6"] {
-  border-left: 2px solid rgba(130, 170, 255, 0.3);
+  border-left: 2px solid color-mix(in srgb, var(--primary, #82aaff) 30%, transparent);
 }
 
 /* Bottom borders (end of each 3x3 box vertically) */
 #sudoku .sudoku-cell[data-r="2"],
 #sudoku .sudoku-cell[data-r="5"],
 #sudoku .sudoku-cell[data-r="8"] {
-  border-bottom: 2px solid rgba(130, 170, 255, 0.3);
+  border-bottom: 2px solid color-mix(in srgb, var(--primary, #82aaff) 30%, transparent);
 }
 
 /* Top borders (start of each 3x3 box vertically) */
 #sudoku .sudoku-cell[data-r="0"],
 #sudoku .sudoku-cell[data-r="3"],
 #sudoku .sudoku-cell[data-r="6"] {
-  border-top: 2px solid rgba(130, 170, 255, 0.3);
+  border-top: 2px solid color-mix(in srgb, var(--primary, #82aaff) 30%, transparent);
 }
 
 /* =========================================================
    User-Entered Cells
    ========================================================= */
 #sudoku .sudoku-cell.user-entered {
-  color: #82aaff; /* Tokyo Night primary blue */
+  color: var(--primary, #82aaff);
   font-weight: 600;
 }
 
@@ -113,41 +120,41 @@
    Active Cell Highlight (Navigation Mode)
    ========================================================= */
 #sudoku .sudoku-cell.active {
-  outline: 2px solid #82aaff; /* Tokyo Night blue */
+  outline: 2px solid var(--primary, #82aaff);
   outline-offset: -2px;
-  background: #3d4269;
+  background: color-mix(in srgb, var(--primary, #82aaff) 20%, var(--muted, #2f334d));
   box-shadow: 
-    0 0 8px rgba(130, 170, 255, 0.4),
-    0 0 12px rgba(130, 170, 255, 0.2),
-    inset 0 0 6px rgba(130, 170, 255, 0.15);
+    0 0 8px color-mix(in srgb, var(--primary, #82aaff) 40%, transparent),
+    0 0 12px color-mix(in srgb, var(--primary, #82aaff) 20%, transparent),
+    inset 0 0 6px color-mix(in srgb, var(--primary, #82aaff) 15%, transparent);
 }
 
 /* =========================================================
    Editing Cell Highlight (Edit Mode)
    ========================================================= */
 #sudoku .sudoku-cell.editing {
-  outline: 2px solid #ffc777; /* Tokyo Night yellow */
+  outline: 2px solid var(--accent, #ffc777);
   outline-offset: -2px;
-  background: #3d3836;
+  background: color-mix(in srgb, var(--accent, #ffc777) 15%, var(--muted, #2f334d));
   box-shadow: 
-    0 0 16px rgba(255, 199, 119, 0.7),
-    0 0 24px rgba(255, 199, 119, 0.4),
-    inset 0 0 12px rgba(255, 199, 119, 0.3);
+    0 0 16px color-mix(in srgb, var(--accent, #ffc777) 70%, transparent),
+    0 0 24px color-mix(in srgb, var(--accent, #ffc777) 40%, transparent),
+    inset 0 0 12px color-mix(in srgb, var(--accent, #ffc777) 30%, transparent);
   animation: editing-cell-glow 1.5s ease-in-out infinite;
 }
 
 @keyframes editing-cell-glow {
   0%, 100% {
     box-shadow: 
-      0 0 16px rgba(255, 199, 119, 0.7),
-      0 0 24px rgba(255, 199, 119, 0.4),
-      inset 0 0 12px rgba(255, 199, 119, 0.3);
+      0 0 16px color-mix(in srgb, var(--accent, #ffc777) 70%, transparent),
+      0 0 24px color-mix(in srgb, var(--accent, #ffc777) 40%, transparent),
+      inset 0 0 12px color-mix(in srgb, var(--accent, #ffc777) 30%, transparent);
   }
   50% {
     box-shadow: 
-      0 0 20px rgba(255, 199, 119, 0.9),
-      0 0 32px rgba(255, 199, 119, 0.6),
-      inset 0 0 16px rgba(255, 199, 119, 0.4);
+      0 0 20px color-mix(in srgb, var(--accent, #ffc777) 90%, transparent),
+      0 0 32px color-mix(in srgb, var(--accent, #ffc777) 60%, transparent),
+      inset 0 0 16px color-mix(in srgb, var(--accent, #ffc777) 40%, transparent);
   }
 }
 
@@ -158,16 +165,16 @@
 #sudoku .sudoku-cell.invalid-move {
   animation: sudoku-invalid-pulse 180ms ease-out;
   box-shadow:
-    0 0 12px rgba(255, 117, 127, 0.85),
-    0 0 24px rgba(255, 117, 127, 0.45);
+    0 0 12px color-mix(in srgb, var(--destructive, #c53b53) 85%, transparent),
+    0 0 24px color-mix(in srgb, var(--destructive, #c53b53) 45%, transparent);
 }
 
 @keyframes sudoku-invalid-pulse {
   0% {
     transform: scale(1.05);
     box-shadow:
-      0 0 16px rgba(255, 117, 127, 0.95),
-      0 0 28px rgba(255, 117, 127, 0.55);
+      0 0 16px color-mix(in srgb, var(--destructive, #c53b53) 95%, transparent),
+      0 0 28px color-mix(in srgb, var(--destructive, #c53b53) 55%, transparent);
   }
   100% {
     transform: none;
@@ -206,11 +213,59 @@
 
 /* Trail effect for moves */
 #sudoku .sudoku-cell.trail {
-  box-shadow: inset 0 0 6px rgba(130, 170, 255, 0.6), 0 0 6px rgba(130, 170, 255, 0.35);
+  box-shadow: 
+    inset 0 0 6px color-mix(in srgb, var(--primary, #82aaff) 60%, transparent), 
+    0 0 6px color-mix(in srgb, var(--primary, #82aaff) 35%, transparent);
   animation: sudoku-trail-fade 220ms ease-out;
 }
 
 @keyframes sudoku-trail-fade {
-  from { background-color: rgba(130, 170, 255, 0.25); }
+  from { background-color: color-mix(in srgb, var(--primary, #82aaff) 25%, transparent); }
   to   { background-color: transparent; }
 }
+
+/* Valid move flash */
+#sudoku .sudoku-cell.valid-move {
+  animation: sudoku-valid-flash 300ms ease-out;
+}
+
+@keyframes sudoku-valid-flash {
+  0% {
+    background: color-mix(in srgb, var(--success, #4fd6be) 40%, transparent);
+    box-shadow:
+      0 0 12px color-mix(in srgb, var(--success, #4fd6be) 80%, transparent),
+      0 0 20px color-mix(in srgb, var(--success, #4fd6be) 50%, transparent),
+      inset 0 0 8px color-mix(in srgb, var(--success, #4fd6be) 35%, transparent);
+  }
+  100% {
+    background: transparent;
+    box-shadow: none;
+  }
+}
+
+/* Conflict highlighting */
+#sudoku .sudoku-cell.conflict {
+  outline: 2px solid var(--destructive, #c53b53);
+  outline-offset: -2px;
+  background: color-mix(in srgb, var(--destructive, #c53b53) 25%, var(--muted, #2f334d));
+  box-shadow:
+    0 0 10px color-mix(in srgb, var(--destructive, #c53b53) 70%, transparent),
+    0 0 18px color-mix(in srgb, var(--destructive, #c53b53) 40%, transparent),
+    inset 0 0 8px color-mix(in srgb, var(--destructive, #c53b53) 30%, transparent);
+  animation: sudoku-conflict-pulse 1s ease-in-out infinite;
+}
+
+@keyframes sudoku-conflict-pulse {
+  0%, 100% {
+    box-shadow:
+      0 0 10px color-mix(in srgb, var(--destructive, #c53b53) 70%, transparent),
+      0 0 18px color-mix(in srgb, var(--destructive, #c53b53) 40%, transparent),
+      inset 0 0 8px color-mix(in srgb, var(--destructive, #c53b53) 30%, transparent);
+  }
+  50% {
+    box-shadow:
+      0 0 14px color-mix(in srgb, var(--destructive, #c53b53) 85%, transparent),
+      0 0 24px color-mix(in srgb, var(--destructive, #c53b53) 55%, transparent),
+      inset 0 0 12px color-mix(in srgb, var(--destructive, #c53b53) 40%, transparent);
+  }
+}
diff --git a/src/useGame/useBoardState.ts b/src/useGame/useBoardState.ts
index c414e0c..8bf2e02 100644
--- a/src/useGame/useBoardState.ts
+++ b/src/useGame/useBoardState.ts
@@ -1,4 +1,4 @@
-import { useState, useCallback } from "react";
+import { useState, useCallback, useMemo } from "react";
 import { sudoku } from "../lib/Sudoku";
 import type { UseCursorType } from "./useCursor";
 
@@ -8,40 +8,190 @@ export function useBoardState(cursorManager: UseCursorType) {
   const [initialBoard, setInitialBoard] = useState<string>(board);
   const [solution, setSolution] = useState<string | null>(null);
 
-  const setDigit = useCallback(
-    (d: string) => {
-      const i = cursor.r * 9 + cursor.c;
-      const arr = board.split("");
-      arr[i] = d;
-      setBoard(arr.join(""));
-    },
-    [cursor, board]
-  );
+  const givenCells = () => {
+    const cells = new Set<number>();
+    for (let i = 0; i < initialBoard.length; i++) {
+      if (initialBoard[i] !== ".") {
+        cells.add(i);
+      }
+    }
+    return cells;
+  };
+
+  const canEditCell = (row: number, col: number): boolean => {
+    const index = row * 9 + col;
+    return !givenCells().has(index);
+  };
+
+  const isValidPlacement = (testBoard: string, row: number, col: number, digit: string): boolean => {
+    if (digit === ".") return true;
+
+    const idx = row * 9 + col;
+
+    for (let c = 0; c < 9; c++) {
+      const checkIdx = row * 9 + c;
+      if (c !== col && testBoard[checkIdx] === digit) {
+        return false;
+      }
+    }
+
+    for (let r = 0; r < 9; r++) {
+      const checkIdx = r * 9 + col;
+      if (r !== row && testBoard[checkIdx] === digit) {
+        return false;
+      }
+    }
+
+    const boxRow = Math.floor(row / 3) * 3;
+    const boxCol = Math.floor(col / 3) * 3;
+    for (let r = boxRow; r < boxRow + 3; r++) {
+      for (let c = boxCol; c < boxCol + 3; c++) {
+        const checkIdx = r * 9 + c;
+        if ((r !== row || c !== col) && testBoard[checkIdx] === digit) {
+          return false;
+        }
+      }
+    }
+
+    return true;
+  };
+
+  const getConflicts = (testBoard: string, row: number, col: number): number[] => {
+    const conflicts: number[] = [];
+    const digit = testBoard[row * 9 + col];
+
+    if (digit === ".") return conflicts;
+
+    for (let c = 0; c < 9; c++) {
+      if (c !== col && testBoard[row * 9 + c] === digit) {
+        conflicts.push(row * 9 + c);
+      }
+    }
+
+    for (let r = 0; r < 9; r++) {
+      if (r !== row && testBoard[r * 9 + col] === digit) {
+        conflicts.push(r * 9 + col);
+      }
+    }
+
+    const boxRow = Math.floor(row / 3) * 3;
+    const boxCol = Math.floor(col / 3) * 3;
+    for (let r = boxRow; r < boxRow + 3; r++) {
+      for (let c = boxCol; c < boxCol + 3; c++) {
+        if ((r !== row || c !== col) && testBoard[r * 9 + c] === digit) {
+          const idx = r * 9 + c;
+          if (!conflicts.includes(idx)) {
+            conflicts.push(idx);
+          }
+        }
+      }
+    }
+
+    return conflicts;
+  };
+
+  const isFullyValid = (testBoard: string): boolean => {
+    for (let r = 0; r < 9; r++) {
+      let rowSum = 0;
+      for (let c = 0; c < 9; c++) {
+        const digit = testBoard[r * 9 + c];
+        if (digit === ".") return false;
+        rowSum += parseInt(digit, 10);
+      }
+      if (rowSum !== 45) return false;
+    }
+
+    for (let c = 0; c < 9; c++) {
+      let colSum = 0;
+      for (let r = 0; r < 9; r++) {
+        const digit = testBoard[r * 9 + c];
+        colSum += parseInt(digit, 10);
+      }
+      if (colSum !== 45) return false;
+    }
+
+    for (let boxRow = 0; boxRow < 3; boxRow++) {
+      for (let boxCol = 0; boxCol < 3; boxCol++) {
+        let boxSum = 0;
+        for (let r = boxRow * 3; r < boxRow * 3 + 3; r++) {
+          for (let c = boxCol * 3; c < boxCol * 3 + 3; c++) {
+            const digit = testBoard[r * 9 + c];
+            boxSum += parseInt(digit, 10);
+          }
+        }
+        if (boxSum !== 45) return false;
+      }
+    }
+
+    return true;
+  };
+
+  const isComplete = (testBoard: string): boolean => {
+    return !testBoard.includes(".");
+  };
+
+  const setDigit = (d: string) => {
+    const i = cursor.r * 9 + cursor.c;
+
+    if (!canEditCell(cursor.r, cursor.c)) {
+      window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
+      return false;
+    }
+
+    const arr = board.split("");
+    arr[i] = d;
+    const newBoard = arr.join("");
+
+    if (!isValidPlacement(newBoard, cursor.r, cursor.c, d)) {
+      const conflicts = getConflicts(newBoard, cursor.r, cursor.c);
+      window.dispatchEvent(
+        new CustomEvent("sudoku-invalid-move", {
+          detail: { row: cursor.r, col: cursor.c, digit: d, conflicts },
+        })
+      );
+      return false;
+    }
+
+    setBoard(newBoard);
+    window.dispatchEvent(
+      new CustomEvent("sudoku-valid-move", {
+        detail: { row: cursor.r, col: cursor.c, digit: d },
+      })
+    );
+    return true;
+  };
+
+  const erase = () => {
+    if (!canEditCell(cursor.r, cursor.c)) {
+      window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
+      return false;
+    }
 
-  const erase = useCallback(() => {
     const i = cursor.r * 9 + cursor.c;
     const arr = board.split("");
     arr[i] = ".";
     setBoard(arr.join(""));
-  }, [cursor, board]);
+    return true;
+  };
 
-  const reset = useCallback(() => {
-    const newBoard = sudoku.generate("medium");
+  const reset = () => {
+    const newBoard = sudoku.generate("easy");
     setBoard(newBoard);
     setInitialBoard(newBoard);
     setSolution(null);
     setCursor({ r: 0, c: 0 });
-  }, [setCursor]);
+  };
 
-  const solveBoard = useCallback(() => {
-    const solved = sudoku.solve(board);
+  const solveBoard = () => {
+    const solved = sudoku.solve(initialBoard);
     if (solved) {
       setSolution(solved);
       setBoard(solved);
+      window.dispatchEvent(new CustomEvent("sudoku-game-won"));
     }
-  }, [board]);
+  };
 
-  const boardToString = useCallback((human: boolean = false) => {
+  const boardToString = (human: boolean = false) => {
     // Computer-optimized: compact 81-character string
     if (!human) {
       return board;
@@ -60,7 +210,7 @@ export function useBoardState(cursorManager: UseCursorType) {
       if (r === 2 || r === 5) result += "------+-------+------\n";
     }
     return result;
-  }, [board]);
+  };
 
   return {
     board,
@@ -71,6 +221,12 @@ export function useBoardState(cursorManager: UseCursorType) {
     reset,
     solveBoard,
     boardToString,
+    canEditCell,
+    isValidPlacement,
+    getConflicts,
+    isFullyValid,
+    isComplete,
+    givenCells,
   };
 }
 
diff --git a/src/useGame/useCursor.ts b/src/useGame/useCursor.ts
index fa24be5..230e508 100644
--- a/src/useGame/useCursor.ts
+++ b/src/useGame/useCursor.ts
@@ -30,6 +30,40 @@ export function useCursor(rendererManager: UseRenderer) {
     setTimeout(() => el.classList.remove("locked-cell"), 300);
   }, [cursor, getCellEl]);
 
+  const flashValid = useCallback(() => {
+    const el = getCellEl(cursor.r, cursor.c);
+    if (!el) return;
+    el.classList.add("valid-move");
+    setTimeout(() => el.classList.remove("valid-move"), 300);
+  }, [cursor, getCellEl]);
+
+  const highlightConflicts = useCallback((conflictIndices: number[]) => {
+    const container = containerRef.current;
+    if (!container) return;
+    const sudokuDiv = container.querySelector("#sudoku");
+    if (!sudokuDiv) return;
+
+    conflictIndices.forEach((idx) => {
+      const r = Math.floor(idx / 9);
+      const c = idx % 9;
+      const el = getCellEl(r, c);
+      if (el) {
+        el.classList.add("conflict");
+      }
+    });
+
+    setTimeout(() => {
+      conflictIndices.forEach((idx) => {
+        const r = Math.floor(idx / 9);
+        const c = idx % 9;
+        const el = getCellEl(r, c);
+        if (el) {
+          el.classList.remove("conflict");
+        }
+      });
+    }, 2000);
+  }, [containerRef, getCellEl]);
+
   const moveCursor = useCallback(
     (dr: number, dc: number, steps = 1) => {
       if (animatingRef.current) return;
@@ -54,9 +88,25 @@ export function useCursor(rendererManager: UseRenderer) {
 
   useEffect(() => {
     const onLockedCell = () => flashLocked();
+    const onValidMove = () => flashValid();
+    const onInvalidMove = (e: Event) => {
+      const detail = (e as CustomEvent).detail;
+      if (detail?.conflicts) {
+        highlightConflicts(detail.conflicts);
+      }
+      flashInvalid();
+    };
+
     window.addEventListener("sudoku-locked-cell", onLockedCell);
-    return () => window.removeEventListener("sudoku-locked-cell", onLockedCell);
-  }, [flashLocked]);
+    window.addEventListener("sudoku-valid-move", onValidMove);
+    window.addEventListener("sudoku-invalid-move", onInvalidMove as EventListener);
+
+    return () => {
+      window.removeEventListener("sudoku-locked-cell", onLockedCell);
+      window.removeEventListener("sudoku-valid-move", onValidMove);
+      window.removeEventListener("sudoku-invalid-move", onInvalidMove as EventListener);
+    };
+  }, [flashLocked, flashValid, flashInvalid, highlightConflicts]);
 
   return {
     cursor,
@@ -64,6 +114,8 @@ export function useCursor(rendererManager: UseRenderer) {
     moveCursor,
     getCellEl,
     flashInvalid,
+    flashValid,
+    highlightConflicts,
   };
 }
 
diff --git a/src/useGame/useGame.ts b/src/useGame/useGame.ts
index 914be9e..6a72ef9 100644
--- a/src/useGame/useGame.ts
+++ b/src/useGame/useGame.ts
@@ -14,7 +14,17 @@ export function useGame(platformHook?: unknown) {
   const gameManager = useGameStatus(rendererManager, cursorManager);
   const { gameStatus, setGameStatus, startGame, stopGame } = gameManager;
   const boardManager = useBoardState(cursorManager);
-  const { board, initialBoard, solution, setDigit, erase, reset, solveBoard } = boardManager;
+  const { 
+    board, 
+    initialBoard, 
+    solution, 
+    setDigit, 
+    erase, 
+    reset, 
+    solveBoard,
+    isComplete,
+    isFullyValid,
+  } = boardManager;
 
   // ---------------------------
   // Keybindings
@@ -37,6 +47,10 @@ export function useGame(platformHook?: unknown) {
   const scoreManager = useScore({
     gameContext: { gameStatus, setGameStatus },
     keystrokeCount: keyLog.length,
+    board,
+    isComplete,
+    isFullyValid,
+    initialBoard,
   });
 
   // ---------------------------
diff --git a/src/useGame/useKeyBindings.ts b/src/useGame/useKeyBindings.ts
index d1670a0..09bdba4 100644
--- a/src/useGame/useKeyBindings.ts
+++ b/src/useGame/useKeyBindings.ts
@@ -78,16 +78,22 @@ export function useKeyBindings(ctx: Ctx) {
         return;
       }
 
-      // R or C to enter edit mode
-      if (k === "r" || k === "R" || k === "c" || k === "C") {
+      // R to replace (only works on user-filled cells)
+      if (k === "r" || k === "R") {
         recordKey(k);
         
-        // Check if current cell is a pre-filled clue (not editable)
         const cellIndex = ctx.cursor.r * 9 + ctx.cursor.c;
         const isClue = ctx.initialBoard[cellIndex] !== ".";
+        const isEmpty = ctx.board[cellIndex] === ".";
         
         if (isClue) {
-          // Flash red - cell is locked
+          // Flash red - cell is locked (pre-filled clue)
+          window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
+          return;
+        }
+        
+        if (isEmpty) {
+          // Flash red - cell is empty (use I to insert)
           window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
           return;
         }
@@ -96,6 +102,47 @@ export function useKeyBindings(ctx: Ctx) {
         return;
       }
 
+      // I to insert (only works on empty cells)
+      if (k === "i" || k === "I") {
+        recordKey(k);
+        
+        const cellIndex = ctx.cursor.r * 9 + ctx.cursor.c;
+        const isClue = ctx.initialBoard[cellIndex] !== ".";
+        const isEmpty = ctx.board[cellIndex] === ".";
+        
+        if (isClue) {
+          // Flash red - cell is locked (pre-filled clue)
+          window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
+          return;
+        }
+        
+        if (!isEmpty) {
+          // Flash red - cell already has a value (use R to replace)
+          window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
+          return;
+        }
+        
+        setEditMode("edit");
+        return;
+      }
+
+      // D or X to delete cell value in navigation mode
+      if (k === "d" || k === "D" || k === "x" || k === "X") {
+        recordKey(k);
+        
+        const cellIndex = ctx.cursor.r * 9 + ctx.cursor.c;
+        const isClue = ctx.initialBoard[cellIndex] !== ".";
+        
+        if (isClue) {
+          // Flash red - can't delete clues
+          window.dispatchEvent(new CustomEvent("sudoku-locked-cell"));
+          return;
+        }
+        
+        ctx.erase();
+        return;
+      }
+
       // Escape cancels count prefix
       if (k === "Escape") {
         recordKey(k);
diff --git a/src/useScore/useScore.ts b/src/useScore/useScore.ts
index 2365d40..a85eae0 100644
--- a/src/useScore/useScore.ts
+++ b/src/useScore/useScore.ts
@@ -3,8 +3,8 @@ import { useTimer } from "./hooks/useTimer";
 import { useScoreTime } from "./useScoreTime";
 import type { GameStatus } from "../types";
 
-const TIME_LIMIT_MS = 60000; // 60 seconds
-const KEYSTROKE_LIMIT = 100;
+const TIME_LIMIT_MS = Infinity; // No time limit by default
+const KEYSTROKE_LIMIT = Infinity; // No keystroke limit by default
 
 type GameScoreContext = {
   gameStatus: GameStatus;
@@ -14,33 +14,68 @@ type GameScoreContext = {
 type UseScoreParams = {
   gameContext: GameScoreContext;
   keystrokeCount: number;
+  board: string;
+  isComplete: (board: string) => boolean;
+  isFullyValid: (board: string) => boolean;
+  initialBoard: string;
 };
 
-export function useScore({ gameContext, keystrokeCount }: UseScoreParams) {
+export function useScore({
+  gameContext,
+  keystrokeCount,
+  board,
+  isComplete,
+  isFullyValid,
+  initialBoard
+}: UseScoreParams) {
   const timer = useTimer();
   const { gameStatus, setGameStatus } = gameContext;
   const { timeValue, startTimer, stopTimer, resetTimer } = timer;
 
   const [finalScore, setFinalScore] = useState<number | null>(null);
 
+  const minMovesRequired = initialBoard.split("").filter(c => c === ".").length;
+  const actualMoves = keystrokeCount;
+  const efficiency = minMovesRequired > 0
+    ? Math.round((actualMoves / minMovesRequired) * 100)
+    : 0;
+
   useScoreTime({ gameStatus, timer });
 
-  // Check for game-over conditions
   useEffect(() => {
     if (gameStatus !== "started") return;
 
-    // Time limit exceeded
+    if (isComplete(board) && isFullyValid(board)) {
+      setGameStatus("game-won");
+      return;
+    }
+
     if (timeValue >= TIME_LIMIT_MS) {
       setGameStatus("game-over");
       return;
     }
 
-    // Keystroke limit exceeded
     if (keystrokeCount >= KEYSTROKE_LIMIT) {
       setGameStatus("game-over");
       return;
     }
-  }, [gameStatus, timeValue, keystrokeCount, setGameStatus]);
+
+    // TODO: Enable efficiency-based game-over in future
+    // if (efficiency > 200) {
+    //   setGameStatus("game-over");
+    //   return;
+    // }
+  }, [gameStatus, timeValue, keystrokeCount, board, setGameStatus, isComplete, isFullyValid, efficiency]);
+
+  useEffect(() => {
+    const handleWin = () => {
+      if (gameStatus === "started") {
+        setGameStatus("game-won");
+      }
+    };
+    window.addEventListener("sudoku-game-won", handleWin);
+    return () => window.removeEventListener("sudoku-game-won", handleWin);
+  }, [gameStatus, setGameStatus]);
 
   useEffect(() => {
     if (gameStatus !== "game-won") return;
@@ -48,10 +83,12 @@ export function useScore({ gameContext, keystrokeCount }: UseScoreParams) {
     const timeInSeconds = timeValue / 1000;
     const baseScore = 100000;
     const timePenalty = Math.floor(timeInSeconds * 10);
-    const score = Math.max(0, baseScore - timePenalty);
+    const extraMoves = Math.max(0, actualMoves - minMovesRequired);
+    const movePenalty = extraMoves * 50;
+    const score = Math.max(0, baseScore - timePenalty - movePenalty);
 
     setFinalScore(score);
-  }, [gameStatus, timeValue]);
+  }, [gameStatus, timeValue, actualMoves, minMovesRequired]);
 
   return {
     timeValue,
@@ -61,6 +98,9 @@ export function useScore({ gameContext, keystrokeCount }: UseScoreParams) {
     finalScore,
     timeLimit: TIME_LIMIT_MS,
     keystrokeLimit: KEYSTROKE_LIMIT,
+    minMovesRequired,
+    actualMoves,
+    efficiency,
   };
 }