 By the end of the Eastern Han dynasty local gentry, clansmen, and villagers built more confined defensive structures in the form of square forts known as "wū bì" (塢壁).These were erected in remote countrysides and had particularly high walls, cornered watchtowers, and gates to the front and back.According to Stephen Turnbull, the "wū bì" are the closest approximation to the concept of a European castle that has ever existed in Chinese history.