castle        
 noun [ C ] uk/ˈkɑː.səl/ us/ˈkæs.əl/
A2 城堡;堡垒
a large strong building, built in the past by a ruler or important person to protect the people inside from attack
  (国际象棋)车(rook的非正式说法)
informal for rook 
 - More examples
 - In addition to his flat in London, he has a villa in Italy and a castle in Scotland.
 - The castle is set on a steep hill.
 - The castle's great hall was decorated with sumptuous wall hangings.
 - The fortifications of the castle were massive and impenetrable.
 - We visited an ancient ruined castle overlooking the sea.
 
Idiom  
castles in the airidiom  空中楼阁;空想,几乎不可能实现的计划
plans that have very little chance of happening
 castle      
verb [ I ]  games specialized uk/ˈkɑː.səl/ us/ˈkæs.əl/
 (国际象棋中)王车易位
in chess, to make a special move that puts your king in a more protected place at the side of the board