Moat

Moats were excavated around castles and other fortifications as part of the defensive system as an obstacle immediately outside the walls. In suitable locations they might be filled with water. A moat made access to the walls difficult for siege weapons, such as siege towers and battering rams, which needed to be brought up against a wall to be effective.

A water-filled moat made the practice of mining, digging tunnels under the castles in order to effect a collapse of the defences, very difficult as well. Segmented moats have one dry section and one section filled with water. Dry moats cut across the narrow part of a spur or peninsula are called neck ditches. Moats separating different elements of a castle, such as the inner and outer wards are cross ditches.

Attribution: https://en.m.wikipedia.org/wiki/Moat