鄂尔多斯高原
By the time the Qin state conquered the other six states and began its reign over a unified China in 221 BC, the nomadic Xiongnu had grown into a powerful invading force in the north and started expanding both east and west. Qin Shi Huang, the first emperor of the Qin dynasty, sent a 300,000-strong army headed by Meng Tian to drive the Xiongnu northward for 1,000 "li" (about 416 km) and began work on what has become known as the Great Wall to guard against invasion. The defensive works he began were said to extend over 10,000 "li" (4,158 km) "from Lintao to Liaodong and even extended across the Yellow River and through Yangshan and Beijia.", that is, from the southwest corner of the Ordos Loop to the Yellow Sea. Yangshan and Beijia are hard to locate, but the wall ran along the Yellow River and included all of the Ordos Loop.