diff --git a/engine/include/bitboard.h b/engine/include/bitboard.h
index 7ac5118..3d940a8 100644
--- a/engine/include/bitboard.h
+++ b/engine/include/bitboard.h
@@ -25,6 +25,39 @@
 #define GET_BIT(bb, sq)     ((bb) & (1ULL << (sq)))
 #define TOGGLE_BIT(bb, sq)  ((bb) ^= (1ULL << (sq)))
 
+// King castling helpers.
+#define BB(sq) (1ULL << (sq))
+
+// a1 = 0, h8 = 63 (rank*8 + file)
+enum Square {
+    A1,B1,C1,D1,E1,F1,G1,H1,
+    A2,B2,C2,D2,E2,F2,G2,H2,
+    A3,B3,C3,D3,E3,F3,G3,H3,
+    A4,B4,C4,D4,E4,F4,G4,H4,
+    A5,B5,C5,D5,E5,F5,G5,H5,
+    A6,B6,C6,D6,E6,F6,G6,H6,
+    A7,B7,C7,D7,E7,F7,G7,H7,
+    A8,B8,C8,D8,E8,F8,G8,H8
+};
+
+#define WK_EMPTY_MASK (BB(F1) | BB(G1))
+#define WQ_EMPTY_MASK (BB(B1) | BB(C1) | BB(D1))
+#define BK_EMPTY_MASK (BB(F8) | BB(G8))
+#define BQ_EMPTY_MASK (BB(B8) | BB(C8) | BB(D8))
+
+// squares king is on / passes / lands (use for attacked-squares test later)
+#define WK_THRU_MASK  (BB(E1) | BB(F1) | BB(G1))
+#define WQ_THRU_MASK  (BB(E1) | BB(D1) | BB(C1))
+#define BK_THRU_MASK  (BB(E8) | BB(F8) | BB(G8))
+#define BQ_THRU_MASK  (BB(E8) | BB(D8) | BB(C8))
+
+// king target squares
+#define WK_TO G1
+#define WQ_TO C1
+#define BK_TO G8
+#define BQ_TO C8
+
+
 enum Color { WHITE = 0, BLACK = 1, BOTH = 2 };
 enum Piece {
     P, N, B, R, Q, K,   // 0..5 white