Sichuan anti-Mongol fortresses

The Sichuan anti-Mongol fortresses are 83 mountain cities built by the soldiers and civilians of the Southern Song Dynasty during the Mongol conquest of China in Sichuan to resist the invasion of the Mongol Empire. Taking full advantage of the geographical advantages of the Sichuan Basin, these fortresses formed a comprehensive three-dimensional defense system, which succeeded in resisting the Mongol attacks for up to 53 years, greatly extending the life of Southern Song.

Source: Wikipedia — Sichuan anti-Mongol fortresses (CC BY-SA 4.0)