A house fortified for defense against an enemy; a fortress. The term seems to include the house and the walls or other works around it. In old writers, the word is used for a town or village fortified.

The house or mansion of a nobleman or prince.

In a ship, there are two parts called by this name; the forecastle, a short deck in the fore part of the ship, above the upper deck; and the hindcastle, at the stern. Castle in the air, a visionary project; a scheme that has no solid foundation.

In the game of chess, to cover the king with a castle, by a certain move.