As you can see from the article, there are many types of fortifications on the wall, all were used to stop the invastion from the northern nomadic tribes in ancient China.

In the Qing Dynasty, China has many ethnic minorities, there will be war between them.Among them, the Huns (which is now in Mongolian) is the North a more powerful minority, they often Manchu will intrusion into the Central Plains.Manchu emperor (king) in order to defend their territory and people, launching a campaign built on such a great project to stop the invasion of the Huns.