Why was the Great Wall of China built?

The purpose of building the Great Wall of China was to protect people and territory from invasion. The walls, together with beacon towers, passes, fortification, etc. created an elaborate defensive system providing Chinese society with a safe and peaceful environment. There were many states, so they built separate sections to prevent invasion from other states. States in the northern part of China built some sections to defend against the northern nomadic tribes. In the Qin Dynasty, the Wall was mainly used to protect the country from nomadic tribes. In the reign of Emperor Wu of Western Han, the Wall played a significant role in protecting the main force as well as being a military base in the wars. Every time the country conquered new territory, the Wall would be extended, so that the local military force would be strengthened. With the help of the Great Wall, the Han Dynasty conquered more and more regions in northwestern China. In the Ming Dynasty, General Qi Jiguang reinforced the sections from Shanhaiguan Pass to Juyongguan Pass by building many watch towers and then trained army based on local landform to defeat invasions.