Yanmen Pass
The Zhao walls to the north were built under King Wuling of Zhao (r. 325–299 BC), whose groundbreaking introduction of nomadic cavalry into his army reshaped Chinese warfare and gave Zhao an initial advantage over his opponents. He attacked the Xiongnu tribes of Linhu () and Loufan () to the north, then waged war on the state of Zhongshan until it was annexed in 296 BC. In the process, he constructed the northernmost fortified frontier deep in nomadic territory. The Zhao walls were dated in the 1960s to be from King Wuling's reign: a southern long wall in northern Henan encompassing the Yanmen Pass; a second line of barricades encircling the Ordos Loop, extending from Zhangjiakou in the east to the ancient fortress of Gaoque () in the Urad Front Banner; and a third, northernmost line along the southern slopes of the Yin Mountains, extending from Qinghe in the east, passing north of Hohhot, and into Baotou.