Gansu's historical position is the channel connecting the western regions. During the Warring States period, Qin, Zhao and Yan were unable to resist the Xiongnu, so they built the Great Wall to block the border and cut off contact with the Xiongnu. Qin Changcheng was built in Longxi County, and the county seat is in Lintao South and Beidi County of Gansu Province today. The Great Wall was built at the border with the nomadic people in the north to protect themselves. Later, many dynasties used it, such as the Great Wall of Han Dynasty, the Great Wall of Northern Wei Dynasty and the Great Wall of Sui Dynasty, as well as the famous military projects in Gansu in the Ming Dynasty. There is also Zhenyuan Bridge, known as the First Bridge of the Yellow River, which is located outside Tongji Gate in the north of Lanzhou. Gansu's military superiority can be proved by two events in the middle of the Tang Dynasty, namely the Anshi Rebellion and the Tubo Rebellion. Two examples show the strategic value of Hexi and Longyou, which had a substantial impact on the fate of the Tang Dynasty.