castle:A large building or set of buildings fortified for defence against an enemy; a fortress, stronghold. Retained as a name for large mansions or country houses, which were formerly feudal castles, but not, like French château, transferred to this sense.——Oxford English Dictionary

castle:(also Castle) a very large strong building, built in the past as a safe place that could be easily defended against attack.——Longman Dictionary of Contemporary English  @en