Castle on a board is a term used to describe the chess piece known as a ROOK. The ROOK is represented by a castle-like structure and is one of the six pieces that make up a chess set. It is the second most powerful piece on the board, and it can move horizontally or vertically across any number of squares. The ROOK is also used to castle, which is a special move that allows the king and ROOK to switch places in order to protect the king from attack.