Castles' barriers are often referred to as moats, which is a five-letter word. Moats are typically deep, wide ditches filled with water that surround a castle and act as a defensive barrier against attackers. They can also be dry ditches or walls, but the most common form of a moat is a water-filled ditch. Moats were used in medieval times to protect castles from invaders and to make it difficult for them to breach the castle walls.