Because Mesopotamia didn't really have many natural barriers that could help keep out intruders, it was prone to attacks. Even if the Mesopotamians built their own defenses, it wouldn't be nearly as as good as a mountain or if they were on a hill. the only natural barrier they had was really the two rivers, Tigris and Euphrates. but since it was near the middle of their land, it really wasn't much of a barrier, unless they were trying to escape to the other side.
