The Xiadu or Lower Capital of Yan (Chinese: 下都; pinyin: Xiàdū) was one of the capitals of Yan during the Warring States period of ancient China.
Xiadu may have been the largest city in the world from 400 to 300 BC, with an estimated peak population over 300,000.
[1] The remains of the city are located in Yi County, Baoding, Hebei Province.
At 30 km2, Xiadu is the largest excavated city from the Warring States period.
A portion of the old city wall, measuring up to 6.8 m high, still remains today.