Guoyu Ancient City is a castle constructed in the early Tang Dynasty (618-907) at the foot of the Taihang Mountains, in an area administered by Yangcheng county in North China's Shanxi province. 

It covers an area of 179,000 square meters, with 1,100 houses and 40 courtyards built during the Ming Dynasty (1368-1644). 

The existing castle wall is 12 meters high, 5 meters wide and 1,400 meters long. It has both residential and defensive functions.