Ancient China's Great Wall was built to prevent people inside from fleeing outward, not merely to keep outsiders from entering. Starting from the Western Han Dynasty, the Great Wall indeed gradually took on an important auxiliary role in "defending internally," especially in preventing the unidirectional flow of population and supplies to the northern nomadic regimes. During Emperor Yuan of Han's reign, the famous memorial "Han Shu · Xiongnu Biography" by Langzhong Hou Ying clearly stated: establishing passes and garrisons was "not only for the Xiongnu," but also to prevent surrendered Hu from thinking of escaping back, to stop the descendants of impoverished soldiers and civilians in border areas from fleeing to relatives among the Xiongnu, or slaves and servants from envying "Xiongnu music" and escaping, as well as thieves/criminals fleeing north to join the enemy. In the late Ming Dynasty, there are also many records: border residents, soldiers, and followers fleeing into the barbarians due to heavy taxes, disasters, and unpaid military wages. The court used the Great Wall's passes, inspections, and prohibitions to restrict this one-way "leakage."