cas ‧ tle W3 /ˈkɑːs ə l $ ˈkæ-/ BrE AmE noun [countable]
[ Date: 1000-1100 ; Language: Old North French ; Origin: castel , from Latin castellum 'building with a defensive wall' , from castrum 'defended place, camp' ]
1 . ( also Castle ) a very large strong building, built in the past as a safe place that could be easily defended against attack:
Edinburgh Castle
a ruined castle
2 . one of the pieces used in a game of ↑ chess . Each player has two castles, which start the game in the corner squares, and can move only forwards or sideways. SYN rook
3 . castles in the air plans or hopes that you have that are unlikely ever to become real