1.A fortified residence, especially that of a prince or nobleman; a fortress. "The house of every one is to him castle and fortress, as well for his defense againts injury and violence, as for his repose." "Our castle's strength Will laugh a siege to scorn." Note: Originally the mediaeval castle was a single strong tower or keep, with a palisaded inclosure around it and inferior buidings, such as stables and the like, and surrounded by a moat; then such a keep or donjon, with courtyards or baileys and accessory buildings of greater elaboration a great hall and a chapel, all surrounded by defensive walls and a moat, with a drawbridge, etc. Afterwards the name was retained by large dwellings that had formerly been fortresses, or by those which replaced ancient fortresses. A Donjon or Keep, an irregular building containing the dwelling of the lord and his family; B C Large round towers ferming part of the donjon and of the exterior; D Square tower, separating the two inner courts and forming part of the donjon; E Chapel, whose apse forms a half-round tower, F, on the exterior walls; G H Round towers on the exterior walls; K Postern gate, reached from outside by a removable fight of steps or inclined plane for hoisting in stores, and leading to a court, L (see small digagram) whose pavement is on a level with the sill of the postern, but below the level of the larger court, with which it communicates by a separately fortified gateway; M Turret, containing spiral stairway to all the stories of the great tower, B, and serving also as a station for signal fire, banner, etc.; N Turret with stairway for tower, C; O Echauguettes; P P P Battlemants consisting of merlons and crenels alternately, the merlons being pierced by loopholes; Q Q Machicolations (those at Q defend the postern K); R Outwork defending the approach, which is a road ascending the hill and passing under all four faces of the castle; S S Wall of the outer bailey. The road of approach enters the bailey at T and passes thence into the castle by the main entrance gateway (which is in the wall between, and defended by the towers, C H) and over two drawbridges and through fortified passages to the inner court.
2.Any strong, imposing, and stately mansion.
3.A small tower, as on a ship, or an elephant's back.
4.A piece, made to represent a castle, used in the game of chess; a rook.
...