A castle is a large, fortified building designed to protect the people inside from attack. Medieval castles had thick stone walls, tall towers for spotting enemies from far away, and narrow windows that archers could shoot through while staying protected. Many castles were surrounded by deep ditches called moats, sometimes filled with water, that made it harder for attackers to reach the walls.

Castles served as both military fortresses and homes for nobles, kings, and their families. Inside a castle, you might find a great hall for feasts and meetings, private chambers, kitchens, storage rooms, and sometimes even dungeons. The largest room was usually the great hall, where the lord held court and hosted important guests.

The word can also describe the rook piece in chess, which looks like a small tower and moves in straight lines across the board. In chess, castling is a special move where the king and rook move at the same time to help protect the king.

People sometimes use “castle” figuratively too. When someone says “a person's home is their castle,” they mean that people should feel safe and in control in their own homes, just as a medieval lord commanded a castle.