This is to prevent the invasion of the Hans. Or tribal GNU C Lyon (Xiongnu) that left only Nouba is unpopular with the early Chinese civilization. Since the Zhou Dynasty (around 400 BC) due to the invasion of China along the northern border. The Qin Dynasty Was ordered to build a wall along the border of Politics. They left only to protect Wakefield and the Turks invaded from the north. After that, there are barriers to many times. But later, it is the nomadic tribes of Mongolia and Manchuria to break through the Great Wall of China is successful.