Once the whole country was unified in Ming Dynasty, Zhu Yuanzhang sent orders to the local governments to build city walls on a large scale. Zhu assumed that “out of all the mountains and rivers in the world, the central Qin is the most strongly fortified and strategically impregnable”. The city wall of Xi'an is an extension of the old Tang Dynasty structure, as a result of this wall building campaign. The city wall, after its extension in the Ming Dynasty, stands 12 meters high. It is 12—14 meters wide across the top, 15---18 meters thick at the bottom and 13.7 kilometers in length. Thus, the Ming Dynasty city wall formed a complex and well-organized system of defense. It is also the most complete city wall that has survived through China's long history.