The Great Wall of China is a series of fortifications made of stone, brick, rammed earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe. Several walls were being built as early as the 7th century BCE; these, later joined together and made bigger and stronger, are now collectively referred to as the Great Wall. https://en.wikipedia.org/wiki/Great_Wall_of_China