Definition of CASTLE for Kids. 1. : a large building or group of buildings usually having high walls with towers that was built in the past to protect against attack. 2. : a large or impressive house.ull Definition of CASTLE. 1. a: a large fortified building or set of buildings b: a massive or imposing house. 2. : a retreat safe against intrusion or invasion. 3. : 3rook. See castle defined for English-language learners. See castle defined for kids.