Diji City, Ancient fortification in Yangcheng County, China.

Diji City (砥洎城), also called Runcheng Xiaocheng, is an oval walled town of about 24,000 square metres built on an earthen mound encircled by the Qin River in Runcheng Village, Yangcheng County, Shanxi. Its walls mix brick and stone with discarded iron-smelting crucibles, and it has a land gate on the south side facing the village and a water gate on the north opening to the river.

Diji City was first built in 1638, the eleventh year of the Ming Chongzhen reign, on a loess rise encircled by the Qin River in Runcheng Village, Yangcheng County, Shanxi. It was also known as Runcheng Xiaocheng, the "small city of Runcheng". In 2006 it was listed in the sixth batch of Major Historical and Cultural Sites Protected at the National Level.

The name Diji comes from Jishui, the old name of the Qin River that encircles the site. Inside the walls stand temples, gardens and courtyard houses whose two- and three-storey pavilion-style buildings visitors can still see today.

Visitors should seek orientation help at the on-site information center since the overall grounds are quite extensive. The area is relatively accessible, though the surrounding water and rocky terrain should be kept in mind when planning your route.

The walls were built with discarded iron smelting crucibles from local foundries, not cooking vessels. These cylindrical clay crucibles were stacked into the rammed-earth and brick masonry, giving the ramparts their particular texture.