Real Dictionary

castle
countable noun: A castle is a large building with thick, high walls. Castles were built by important people, such as kings, in former times, especially for protection during wars and battles.
countable noun: In chess, a castle is a piece that can be moved forwards, backwards, or sideways.

castle in American English
a large building or group of buildings fortified with thick walls, battlements, and often a moat; castles were the strongholds of noblemen in the Middle Ages
to castle a king
to put into, or furnish with, a castle
intransitive verb: to move the king two squares horizontally and bring the appropriate rook to the square the king has passed over
noun: a fortified, usually walled residence, as of a prince or noble in feudal times
transitive verb: to place or enclose in or as in a castle

castle in British English
noun: a fortified building or set of buildings, usually permanently garrisoned, as in medieval Europe