The term 'PART OF A CASTLE' can be broken down into two crossword solutions: KEEP (4 letters) and TOWER (5 letters). A KEEP is a fortified TOWER within a castle, usually located at the center of the castle. It was used as a last line of defense and was often the residence of the lord of the castle. A TOWER is a tall structure, usually made of stone or brick, that is part of a castle's defensive walls. Towers were used to provide a vantage point for archers and other defenders.