Definition of "Castle" :

noun: interchanging the positions of the king and a rook

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

noun: a large and stately mansion

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