n. A building, or series of connected buildings, fortified for defense against an enemy; a fortified residence; a fortress.n. In heraldry, a representation of two or more towers connected by curtains, often having a gateway in one of the curtains, and always embattled.n. The house or mansion of a person of rank or wealth: somewhat vaguely applied, but usually to a large and more or less imposing building.n. A piece made in the form of a castle, donjon, or tower, used in the game of chess; the rook.n. A kind of helmet.n. Nautical, a kind of fighting-tower formerly erected on war-galleys, etc., near the bow and stern, and called respectively forecastle and aftcastle. See cut under cadenas.n. Synonyms See fortification.In chess, to move the king from his own square two squares to the right or left, and bring the rook or castle to the square the king has passed over.