When did they start building the Great Wall of China ?
Conversely , the Tang ( 618–907 ) , the Song ( 960–1279 ) , the Yuan ( 1271–1368 ) , and the Qing ( 1644–1911 ) mostly left the Walls to rot , having resolved the Inner Asian threat through military campaigns and diplomacy ( the Tang , Yuan , and Qing ) , or were simply took weak to build walls ( the Song ) .