[count] 1 : a large building usually with high, thick walls and towers that was built in the past to protect against attack2 informal : a large expensive house
▪ Millionaires built their castles along the lake.3 : a piece in the game of chess that looks like a castle tower : rook — see picture at chess
▪ Millionaires built their castles along the lake.
castle in the air also castle in Spain
: a dream, plan, or idea that can never become real — usually plural ▪ He's just building castles in the air if he thinks he can open a restaurant without any money.