Between the fall of the Western Han Dynasty and the middle period of the Tang Dynasty,the aristocratic families fortified their manors in the countryside.These forts are called '塢堡'.When the barbarians overran northern China when Jin rule in the north collapsed,the aristocrats remaining in the north and their tenants found refuge in these forts and managed to stop the barbarians from simply just having them massacred.Due to the prevalence of such forts,the barbarians couldn't clear out the aristocrats and eventually simply compromised with them.
Here are some models of such forts:
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