The city has three land walls along its western boundary, each 4 miles long. The outermost barrier is actually a breastwork just inside a deep, wide trench that serves as a moat. The next barrier, known as the outer wall, lies 40 to 50 feet behind the breastwork. This wall, which rises about 25 feet high, sits atop a 10-foot embankment. The inner wall, which is about 40 feet high, is 40 to 60 feet behind the outer wall. Towers stand along the outer and inner walls, enabling defenders to shoot at attackers.