Castling is a defensive supermove. It is the only occasion when you move two pieces and count it as one move. Castling involves the king and one of the rooks. The player may castle either to the left with the left rook, or to the right with the right rook.

Three conditions must be met to castle on the chess board:
• The king must not be in check.
• The king and the castling rook must not have been moved.
• If castling-right, the hex between them must be vacant and unthreatened.

Once the above conditions have been met, castle-right by moving the king next to the rook, then moving the rook to the opposite side of the king. Castle-left by swapping the positions of the rook and king. (See below.)

before castling on the 4P3 chess board
Before castling on the chess board:
White and Black are ready to castle to their right. Red is ready to castle to Red's left.

castling on the 4P3 chess board

White castles to the right; Black does the same:
.....1) King moves next to the rook.
.....2) Rook moves to the opposite side of the king.
Red castles to the left: King and rook swap positions.