Xi'an is one of the few cities in China where old city walls are still visible.The city wall of Xi'an is an extension of the old Tang Dynasty structure, as a result of this wall building campaign. Xi'an's city wall after its enlargement in the Ming Dynasty stands 12 meters high. It is 12-14 meters across the top, 15-18 meters thick at bottom and 13.7 kilometers in length. There is a rampart every 120 meters. The ramparts are towers that extend out from the main wall, the top of the rampart being at the same level as the top of the wall.  The distance between two ramparts is just within the range of arrow shot from either side. This allowed soldiers to protect the entire wall without exposing themselves to the enemy. There are altogether 98 of them on the wall; each has a sentry building on top of it. The weapons in ancient times were primitive. The gates of the city wall were the only way to go into and out of town. Therefore, these gates were important and strategic points, that the feudal rulers racked their brains to try to defend.