The moat can greatly hinder troop movement. In order to dig, first a trail has to be marked along a desired path, then available foot soldiers must be commanded to dig it. Digging the moat takes considerable time, since progress is done tile by tile. Units can also dig out friendly players' moats. Trees or vegetation cannot be dug under, as well as no moat can be sited on elevated ground.
When sieging, hostile units can also fill in the moat, creating sandy terrain in their wake. While filling is done much faster than digging out the line, it comes with a risk however: digging units have a chance of "drowning" when moving to a new tile.