It's spelled "Moat".
Moats are basically big trenches around castles/fortresses filled with water, to prevent some siege objects like ladders from easily being able to reach the walls, and totally prevent infantry from going against the wall to take cover from the archers.
I don't think it will technically be possible in LiF, since water isn't handled in a dynamic way. Water is considered kind of like bedrock in minecraft.. it is all at the same layer in the whole world, no matter where you dig. So, you COULD make a moat, but it would quite useless, water would be at the very bottom of it, and there wouldn't be much water, as you can't dig under it.