Why was the Great Wall of China built?

Like many walls built around a country or a city, the wall served two main purposes. One purpose of the Great Wall of China was to keep people in China. The wall controlled how many people were both entering the country and fleeing to other areas. The second purpose of the Great Wall of China was to protect China from invaders from the north. In this case, those specific invaders were most often the Mongols. Despite the Great Wall of China being huge, the Mongols did eventually conquer China. When the wall was originally started, it was started in sections as defense against warring feudal states. Eventually, Qun Shi Huang united different parts of the country into a single empire, and he then had the separate wall sections joined together to fend off invasions from the north.