Whose raids did the Great Wall of China protect?

Around two hundred BC. in China, the Great Wall of China was erected. This massive structure on the border with a length of about 5 thousand kilometers served mainly as a defense against the nomadic tribes of the Huns. Also, stone fortifications fixed the territory of the state and did not allow local tribes to cross the border and merge with the nomads of the north. The thickness of the Great Wall of China is about 6 meters. The strengthening did not allow the possibility of migration of local residents to the vicinity of other states.