Castle defense is a term used to describe the various methods used to protect a castle from attack. One of the most common methods of castle defense is the DRAWBRIDGE, which is a bridge that can be raised or lowered to allow access to the castle. A MOAT is another form of castle defense, which is a deep, wide ditch filled with water that surrounds the castle walls. Finally, a PARAPET is a low wall built along the edge of a castle's roof or walls, providing protection for those inside.