What is the famous defensive wall system in northern China called?

The Great Wall of China is the famous defensive wall system in northern China.

The Great Wall is not one continuous wall built at one time. It is a network of walls, fortifications, watchtowers and barriers constructed and rebuilt by different Chinese states and dynasties over many centuries.

The Qin dynasty connected earlier defensive structures in the third century BCE, while many of the best-preserved sections were built during the Ming dynasty between the 14th and 17th centuries. The wall helped control movement, protect agricultural regions and support border defense.

Contrary to a popular myth, the Great Wall is not reliably visible from the Moon with the unaided eye. Its total measured length, including trenches and natural barriers, is more than 21,000 kilometres, though conditions vary widely along the route.