moat 
(mōt)
(mōt)n.
1. A deep wide ditch, usually filled with water, typically surrounding a fortified medieval town, fortress, or castle as a protection against assault.
2. A ditch similar to one surrounding a fortification: A moat separates the animals in the zoo from the spectators.
tr.v. moat·ed, moat·ing, moats
To surround with or as if with a moat.
[Middle English mote, mound, moat (since both mounds and moats form part of fortifications), from Old French, mound; akin to Medieval Latin mota, perhaps of Germanic origin and akin to English MUD.]