a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and often a moat:Edinburgh Castlethe crumbling stonework of a ruined castle
a magnificent and imposing old mansion: [in names]:Castle Howard
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.
[with object] move (the king) by castling.
castles in the air (or in Spain)
visionary unattainable schemes; daydreams:my father built castles in the air about owning a boat
