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.
• The hex or hexes between them must be vacant and unthreatened.

before and after castling left
Castling left on the chess board.

before and after castling right
Castling right on the chess board.