The Huns, who were pushed out of the region north of China, and went on to give the Roman empire a run for its money.
Imagine being the lowly soldiers picked to man that outpost! They must have been the total screw-ups of their day!............
Actually, these guys were pros, which is how these structures got built. They were expensive to recruit, train and maintain (as opposed to the traditional farmer conscripts), and the cost of their upkeep possibly played a part in the eventual collapse of the dynasty. I read a (U of Eastern Illinois?) monograph about the subject that estimated that the annual cost of this 200K+, mostly cavalry, force aimed at quelling the persistent Hun threat ate up several times the annual tax revenues of the empire, resulting in several rounds of tax hikes, aristocratic as well as peasant rebellions and the systematic devaluation of the currency (via adulteration with base metals). The Han Empire's rulers were damned if they did (counter Hun cavalry raids deep inside the empire by raising an expensive professional, mostly cavalry army), and damned if they didn't (thereby subjecting themselves to ever-bolder and more damaging raids).
Sun Tzu - The Art of War
They must not have read this part:
1. Sun Tzu said: Raising a host of a hundred thousand men and marching them great distances entails heavy loss on the people and a drain on the resources of the State. The daily expenditure will amount to a thousand ounces of silver. There will be commotion at home and abroad, and men will drop down exhausted on the highways. As many as seven hundred thousand families will be impeded in their labor. -