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

Edinburgh Castle

the crumbling stonework of a ruined castle

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