A moat's a deep, broad trench, usually filled with water, that surrounds a structure, installation, or town, historically to provide it with a preliminary line of defense. Historic uses Traditionally, moats were excavated around castles and fortifications as part of the defensive system, and were usually filled with water. They provided a preliminary barrier outside the fortification walls against attacks upon the complex. A moat made access to the walls difficult for siege weapons, such as sie… (More on Moat)