Castle defense

MOAT

This water-filled trench surrounds a fortress to slow down attackers and was often paired with a drawbridge. Think of a deep barrier that’s not made of stone or metal.

A moat is a deep, wide ditch usually filled with water that surrounds a castle to protect it from attacks. The clue "castle defense" directly points to this feature since moats were a common defensive measure in medieval fortifications. There’s no wordplay here—it’s a straightforward definition where the answer fits the clue exactly.