diff --git a/packages/chess/src/rules/castling.test.ts b/packages/chess/src/rules/castling.test.ts
new file mode 100644
index 0000000..5548b3b
--- /dev/null
+++ b/packages/chess/src/rules/castling.test.ts
@@ -0,0 +1,242 @@
+import { describe, it, expect } from "vitest";
+import { Session } from "@paratype/rete";
+import type { EntityId } from "@paratype/rete";
+import { getCastlingMoves, applyCastlingMove, type IsSquareAttacked } from "./castling.js";
+
+const mkId = (n: number) => n as EntityId;
+
+function insertPiece(
+  session: Session,
+  id: number,
+  type: string,
+  color: string,
+  square: number,
+  hasMoved = false,
+): EntityId {
+  const eid = mkId(id);
+  session.insert(eid, "PieceType", type);
+  session.insert(eid, "Color", color);
+  session.insert(eid, "Position", square);
+  session.insert(eid, "HasMoved", hasMoved);
+  return eid;
+}
+
+/** Minimal castling position: a king on its home square plus both home rooks. */
+function setupCastlingPosition(color: "white" | "black"): {
+  session: Session;
+  kingId: EntityId;
+  kingsideRook: EntityId;
+  queensideRook: EntityId;
+} {
+  const session = new Session({ autoFire: false });
+  const backRank = color === "white" ? 0 : 56;
+  const kingId = insertPiece(session, 1, "king", color, backRank + 4);
+  const kingsideRook = insertPiece(session, 2, "rook", color, backRank + 7);
+  const queensideRook = insertPiece(session, 3, "rook", color, backRank + 0);
+  return { session, kingId, kingsideRook, queensideRook };
+}
+
+describe("getCastlingMoves - white", () => {
+  it("both castling moves available when path is clear", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves).toHaveLength(2);
+    expect(moves.some(m => m.side === "kingside")).toBe(true);
+    expect(moves.some(m => m.side === "queenside")).toBe(true);
+  });
+
+  it("kingside target is g1 (6); queenside target is c1 (2)", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const moves = getCastlingMoves(session, kingId);
+    const ks = moves.find(m => m.side === "kingside");
+    const qs = moves.find(m => m.side === "queenside");
+    expect(ks?.to).toBe(6);
+    expect(ks?.rookFrom).toBe(7);
+    expect(ks?.rookTo).toBe(5);
+    expect(qs?.to).toBe(2);
+    expect(qs?.rookFrom).toBe(0);
+    expect(qs?.rookTo).toBe(3);
+  });
+
+  it("kingside castling only when queenside path blocked by piece on b1", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    insertPiece(session, 10, "knight", "white", 1); // b1 blocks queenside
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves).toHaveLength(1);
+    expect(moves[0]?.side).toBe("kingside");
+  });
+
+  it("queenside castling only when kingside path blocked by piece on f1", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    insertPiece(session, 10, "bishop", "white", 5); // f1 blocks kingside
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves).toHaveLength(1);
+    expect(moves[0]?.side).toBe("queenside");
+  });
+
+  it("enemy piece between king and rook also blocks castling", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    insertPiece(session, 10, "knight", "black", 6); // g1
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+  });
+
+  it("no castling after king has moved", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    session.insert(kingId, "HasMoved", true);
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves).toHaveLength(0);
+  });
+
+  it("no kingside castling when kingside rook has moved (but queenside still ok)", () => {
+    const { session, kingId, kingsideRook } = setupCastlingPosition("white");
+    session.insert(kingsideRook, "HasMoved", true);
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+    expect(moves.some(m => m.side === "queenside")).toBe(true);
+  });
+
+  it("no castling when no rook present on rook-home square", () => {
+    const { session, kingId, kingsideRook } = setupCastlingPosition("white");
+    session.retract(kingsideRook, "Position");
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+  });
+
+  it("rejects castling when king is currently in check", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const isAttacked: IsSquareAttacked = (_s, sq) => sq === 4; // e1 attacked
+    const moves = getCastlingMoves(session, kingId, isAttacked);
+    expect(moves).toHaveLength(0);
+  });
+
+  it("rejects castling through check (king passes attacked square)", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const isAttacked: IsSquareAttacked = (_s, sq) => sq === 5; // f1 attacked
+    const moves = getCastlingMoves(session, kingId, isAttacked);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+    // Queenside (transit d1=3, c1=2) unaffected.
+    expect(moves.some(m => m.side === "queenside")).toBe(true);
+  });
+
+  it("rejects castling into check (destination attacked)", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const isAttacked: IsSquareAttacked = (_s, sq) => sq === 6; // g1 attacked
+    const moves = getCastlingMoves(session, kingId, isAttacked);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+  });
+
+  it("b1 being attacked does NOT prevent queenside castling (king doesn't cross b1)", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const isAttacked: IsSquareAttacked = (_s, sq) => sq === 1; // b1 attacked
+    const moves = getCastlingMoves(session, kingId, isAttacked);
+    expect(moves.some(m => m.side === "queenside")).toBe(true);
+  });
+
+  it("attacker color passed to callback is the opposing color", () => {
+    const { session, kingId } = setupCastlingPosition("white");
+    const seenColors: string[] = [];
+    const spy: IsSquareAttacked = (_s, _sq, byColor) => {
+      seenColors.push(byColor);
+      return false;
+    };
+    getCastlingMoves(session, kingId, spy);
+    expect(seenColors.every(c => c === "black")).toBe(true);
+  });
+
+  it("non-rook piece on rook home square blocks that side", () => {
+    const { session, kingId, kingsideRook } = setupCastlingPosition("white");
+    // Replace the kingside rook with a queen (same square, same color).
+    session.insert(kingsideRook, "PieceType", "queen");
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+    expect(moves.some(m => m.side === "queenside")).toBe(true);
+  });
+
+  it("returns empty list when pieceId has no Position/Color", () => {
+    const session = new Session({ autoFire: false });
+    expect(getCastlingMoves(session, mkId(999))).toEqual([]);
+  });
+
+  it("king not on home square yields no castling moves", () => {
+    const session = new Session({ autoFire: false });
+    // King on e2 (12) — HasMoved still false but geometry is wrong.
+    insertPiece(session, 1, "king", "white", 12);
+    insertPiece(session, 2, "rook", "white", 7);
+    insertPiece(session, 3, "rook", "white", 0);
+    expect(getCastlingMoves(session, mkId(1))).toHaveLength(0);
+  });
+});
+
+describe("getCastlingMoves - black", () => {
+  it("both black castling moves available when path clear", () => {
+    const { session, kingId } = setupCastlingPosition("black");
+    const moves = getCastlingMoves(session, kingId);
+    expect(moves).toHaveLength(2);
+  });
+
+  it("black kingside target is g8 (62); queenside target is c8 (58)", () => {
+    const { session, kingId } = setupCastlingPosition("black");
+    const moves = getCastlingMoves(session, kingId);
+    const ks = moves.find(m => m.side === "kingside");
+    const qs = moves.find(m => m.side === "queenside");
+    expect(ks?.to).toBe(62);
+    expect(ks?.rookFrom).toBe(63);
+    expect(ks?.rookTo).toBe(61);
+    expect(qs?.to).toBe(58);
+    expect(qs?.rookFrom).toBe(56);
+    expect(qs?.rookTo).toBe(59);
+  });
+
+  it("black rejects castling through check on f8", () => {
+    const { session, kingId } = setupCastlingPosition("black");
+    const isAttacked: IsSquareAttacked = (_s, sq) => sq === 61; // f8 attacked
+    const moves = getCastlingMoves(session, kingId, isAttacked);
+    expect(moves.some(m => m.side === "kingside")).toBe(false);
+  });
+
+  it("black attacker color passed to callback is white", () => {
+    const { session, kingId } = setupCastlingPosition("black");
+    const seenColors: string[] = [];
+    const spy: IsSquareAttacked = (_s, _sq, byColor) => {
+      seenColors.push(byColor);
+      return false;
+    };
+    getCastlingMoves(session, kingId, spy);
+    expect(seenColors.every(c => c === "white")).toBe(true);
+  });
+});
+
+describe("applyCastlingMove", () => {
+  it("moves white king to g1 and kingside rook to f1", () => {
+    const { session, kingId, kingsideRook } = setupCastlingPosition("white");
+    const moves = getCastlingMoves(session, kingId);
+    const kingside = moves.find(m => m.side === "kingside");
+    expect(kingside).toBeDefined();
+    applyCastlingMove(session, kingside!);
+    expect(session.get(kingId, "Position")).toBe(6);
+    expect(session.get(kingsideRook, "Position")).toBe(5);
+    expect(session.get(kingId, "HasMoved")).toBe(true);
+    expect(session.get(kingsideRook, "HasMoved")).toBe(true);
+  });
+
+  it("moves white king to c1 and queenside rook to d1", () => {
+    const { session, kingId, queensideRook } = setupCastlingPosition("white");
+    const moves = getCastlingMoves(session, kingId);
+    const queenside = moves.find(m => m.side === "queenside");
+    expect(queenside).toBeDefined();
+    applyCastlingMove(session, queenside!);
+    expect(session.get(kingId, "Position")).toBe(2);
+    expect(session.get(queensideRook, "Position")).toBe(3);
+  });
+
+  it("moves black king to g8 and kingside rook to f8", () => {
+    const { session, kingId, kingsideRook } = setupCastlingPosition("black");
+    const moves = getCastlingMoves(session, kingId);
+    const kingside = moves.find(m => m.side === "kingside");
+    expect(kingside).toBeDefined();
+    applyCastlingMove(session, kingside!);
+    expect(session.get(kingId, "Position")).toBe(62);
+    expect(session.get(kingsideRook, "Position")).toBe(61);
+  });
+});
diff --git a/packages/chess/src/rules/castling.ts b/packages/chess/src/rules/castling.ts
new file mode 100644
index 0000000..c80d13f
--- /dev/null
+++ b/packages/chess/src/rules/castling.ts
@@ -0,0 +1,180 @@
+/**
+ * Castling rules for @paratype/chess.
+ *
+ * FIDE castling conditions:
+ *   1. King has NOT moved (HasMoved === false)
+ *   2. The relevant Rook has NOT moved (HasMoved === false)
+ *   3. No pieces between king and rook
+ *   4. King is NOT currently in check
+ *   5. King does NOT pass through an attacked square
+ *   6. King does NOT land on an attacked square
+ *
+ * Conditions 4-6 require check-detection (P2.18). We accept an `isSquareAttacked`
+ * callback so that this module can be wired up once check-detection lands. By
+ * default the callback treats all squares as un-attacked, so Phase 2 pre-integration
+ * only enforces conditions 1-3.
+ */
+import type { Session, EntityId } from "@paratype/rete";
+import type { PieceColor, Square } from "../schema.js";
+import { oppositeColor } from "../schema.js";
+import type { LegalMove } from "./types.js";
+import {
+  getPiecePosition,
+  getPieceColor,
+  isPieceAt,
+  getPieceAt,
+} from "./board-queries.js";
+
+/** Callback that reports whether `square` is attacked by `byColor`. */
+export type IsSquareAttacked = (
+  session: Session,
+  square: Square,
+  byColor: PieceColor,
+) => boolean;
+
+/** Default: no squares ever attacked (used until P2.18 integrates check detection). */
+const neverAttacked: IsSquareAttacked = () => false;
+
+interface CastlingConfig {
+  /** King's starting square (must match current king position). */
+  readonly kingFrom: Square;
+  /** King's destination square. */
+  readonly kingTo: Square;
+  /** Rook's starting square. */
+  readonly rookFrom: Square;
+  /** Rook's destination square. */
+  readonly rookTo: Square;
+  /** Squares between king and rook that must be empty. */
+  readonly between: ReadonlyArray<Square>;
+  /** Squares the king passes through (including destination) — must not be attacked. */
+  readonly kingTransit: ReadonlyArray<Square>;
+}
+
+const CASTLING_CONFIGS: Record<
+  PieceColor,
+  { readonly kingside: CastlingConfig; readonly queenside: CastlingConfig }
+> = {
+  white: {
+    kingside: {
+      kingFrom: 4, kingTo: 6, rookFrom: 7, rookTo: 5,
+      between: [5, 6], kingTransit: [5, 6],
+    },
+    queenside: {
+      kingFrom: 4, kingTo: 2, rookFrom: 0, rookTo: 3,
+      between: [1, 2, 3], kingTransit: [3, 2],
+    },
+  },
+  black: {
+    kingside: {
+      kingFrom: 60, kingTo: 62, rookFrom: 63, rookTo: 61,
+      between: [61, 62], kingTransit: [61, 62],
+    },
+    queenside: {
+      kingFrom: 60, kingTo: 58, rookFrom: 56, rookTo: 59,
+      between: [57, 58, 59], kingTransit: [59, 58],
+    },
+  },
+};
+
+/** A castling move — extends LegalMove with rook-move metadata. */
+export interface CastlingMove extends LegalMove {
+  readonly isCastling: true;
+  readonly rookFrom: Square;
+  readonly rookTo: Square;
+  readonly side: "kingside" | "queenside";
+}
+
+/**
+ * Return all currently-legal castling moves for `kingId` (0, 1 or 2).
+ *
+ * Enforces FIDE conditions 1-3 unconditionally. Conditions 4-6 are only enforced
+ * when a non-default `isSquareAttacked` callback is supplied.
+ */
+export function getCastlingMoves(
+  session: Session,
+  kingId: EntityId,
+  isSquareAttacked: IsSquareAttacked = neverAttacked,
+): CastlingMove[] {
+  const from = getPiecePosition(session, kingId);
+  const color = getPieceColor(session, kingId);
+  if (from === null || color === null) return [];
+
+  // Condition 1: King must not have moved.
+  if (session.get(kingId, "HasMoved") !== false) return [];
+
+  const configs = CASTLING_CONFIGS[color];
+  const enemyColor = oppositeColor(color);
+  const moves: CastlingMove[] = [];
+
+  const entries: ReadonlyArray<readonly ["kingside" | "queenside", CastlingConfig]> = [
+    ["kingside", configs.kingside],
+    ["queenside", configs.queenside],
+  ];
+
+  for (const [side, cfg] of entries) {
+    // King must currently stand on the castling origin.
+    if (from !== cfg.kingFrom) continue;
+
+    // Condition 2a: Rook must exist at rookFrom.
+    const rookId = getPieceAt(session, cfg.rookFrom);
+    if (rookId === null) continue;
+
+    // Verify the piece at rookFrom actually is a friendly rook.
+    const rookType = session.get(rookId, "PieceType");
+    const rookColor = session.get(rookId, "Color");
+    if (rookType !== "rook" || rookColor !== color) continue;
+
+    // Condition 2b: Rook must not have moved.
+    if (session.get(rookId, "HasMoved") !== false) continue;
+
+    // Condition 3: All squares between king and rook must be empty.
+    let blocked = false;
+    for (const sq of cfg.between) {
+      if (isPieceAt(session, sq)) { blocked = true; break; }
+    }
+    if (blocked) continue;
+
+    // Condition 4: King must not currently be in check.
+    if (isSquareAttacked(session, from, enemyColor)) continue;
+
+    // Conditions 5 & 6: Every square the king passes through (and lands on) must be safe.
+    let passesAttackedSquare = false;
+    for (const sq of cfg.kingTransit) {
+      if (isSquareAttacked(session, sq, enemyColor)) {
+        passesAttackedSquare = true;
+        break;
+      }
+    }
+    if (passesAttackedSquare) continue;
+
+    moves.push({
+      pieceId: kingId,
+      from: cfg.kingFrom,
+      to: cfg.kingTo,
+      isCapture: false,
+      isCastling: true,
+      rookFrom: cfg.rookFrom,
+      rookTo: cfg.rookTo,
+      side,
+    });
+  }
+
+  return moves;
+}
+
+/**
+ * Apply a castling move: move both king and rook, and mark both as having moved.
+ * Caller is responsible for passing a `CastlingMove` returned by `getCastlingMoves`.
+ */
+export function applyCastlingMove(session: Session, move: CastlingMove): void {
+  const rookId = getPieceAt(session, move.rookFrom);
+  if (rookId === null) return;
+
+  // Move king.
+  session.insert(move.pieceId, "Position", move.to);
+  session.insert(move.pieceId, "HasMoved", true);
+
+  // Move rook.
+  session.insert(rookId, "Position", move.rookTo);
+  session.insert(rookId, "HasMoved", true);
+}