noun

a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and in many cases a moat.

Edinburgh Castle

fortress,fort,stronghold,fortification,keep,citadel,fastness,tower,peel,palace,chateau,donjon,alcazar

verb

make a special move (no more than once in a game by each player) in which the king is transferred from its original square two squares along the back rank towards a rook on its corner square which is then transferred to the square passed over by the king.

both of the players castled on the queenside