Simply put, the Great Wall of China (as we know it today) didn't exist for two millennia. What many people now see as the Great Wall of China was built by the

Simply put, the Great Wall of China (as we know it today) didn't exist for two millennia. What many people now see as the Great Wall of China was built by the Ming dynasty in the late 1400s and lasted at most 200 years before the Ming dynasty fell (they were still constructing it when the Ming dynasty fell, and the subsequent Qing dynasty declined to continue building it). Before then, the walled fortifications were generally more sporadic, more temporary, and definitely more lightly manned. It would have required much fewer men to defend it--probably looking at say 20k for the Qin or Han era walls--and the Chinese state was, well, a state during much of this time period (by Weber's definition, that it has a monopoly on the legitimate use of force), unlike most of Medieval Europe. Even then, the Chinese state failed several times over the course of two millennia, and a few of those times were quite literally people manning faraway places going "why are we doing this?" and sacking the capital.