In imperial times, Gansu was an important strategic outpost and communications link for the Chinese empire, as the Hexi Corridor runs along the "neck" of the province. The Han dynasty extended the Great Wall across this corridor, building the strategic Yumenguan (Jade Gate Pass, near Dunhuang) and Yangguan fort towns along it. Remains of the wall and the towns can be found there. The Ming dynasty built the Jiayuguan outpost in Gansu. To the west of Yumenguan and the Qilian Mountains, at the northwestern end of the province, the Yuezhi, Wusun, and other nomadic tribes dwelt (Shiji 123), occasionally figuring in regional imperial Chinese geopolitics.