What is the longest defensive wall in the world, built across northern China?

The Great Wall of China is the longest defensive wall in the world.

The Great Wall is a connected system of walls, fortifications, watchtowers, passes, and related defenses built and rebuilt over many centuries. Its officially surveyed total length, including trenches and natural barriers, is about 21,196 kilometres. The surviving wall sections are not one continuous structure from a single period.

Early walls were constructed by states before China’s imperial unification. Qin Shi Huang connected and expanded earlier defenses in the third century BCE, while the Ming dynasty built many of the stone and brick sections most visited today. Those Ming defenses were designed primarily to protect against northern raids.

Contrary to a popular myth, the wall is not plainly visible from the Moon with the unaided eye. It also did not stop every invasion. Its importance lay in surveillance, communications, troop movement, and controlling passages through difficult terrain.