Most of the Great Wall, the connected fortification system, is located in China.
The Great Wall is not one uninterrupted structure built at one time. It is a network of walls, trenches, passes, watchtowers, and natural barriers constructed, rebuilt, and linked by successive Chinese states over more than two thousand years. The best-known surviving sections were built or strengthened during the Ming dynasty, from the 14th to the 17th centuries.
The wall's military purpose was to help control movement, protect agricultural regions, and warn of approaching forces. It stretched across northern China, following ridges and difficult terrain. The Ming sections were often faced with bricks and stone, while earlier walls frequently used rammed earth.
The popular claim that the Great Wall is visible from the Moon with the unaided eye is false. Its length also depends on what is counted: including branches and associated defenses, official Chinese surveys have measured more than 21,000 kilometres. The wall is a cultural monument, not a single perfectly continuous barrier.