castle

(noun) a large and stately mansion. Synonyms: palace.

(noun) a large building formerly occupied by a ruler and fortified against attack.

(noun) (chess) the piece that can move any number of unoccupied squares in a direction parallel to the sides of the chessboard. Synonyms: rook.

(noun) interchanging the positions of the king and a rook. Synonyms: castling.

(verb) move the king two squares toward a rook and in the same move the rook to the square next past the king.