The term 'CASTLE GUARD' can be broken down into two words, 'CASTLE' and 'GUARD'. The first word, 'CASTLE', is a large fortified structure that was built to protect its inhabitants from attack. The second word, 'GUARD', refers to someone who is responsible for protecting the CASTLE and its inhabitants. The crossword solution for the term 'CASTLE GUARD' is 'MOAT', which is a deep, wide ditch filled with water that surrounds a CASTLE and serves as an additional layer of defense.