What is the longest known defensive wall system in the world, extending over 21,000 kilometres in China?

The Great Wall of China is the world’s longest known defensive wall system, with combined sections measuring more than 21,000 kilometres.

It is not one continuous wall built at one time. Chinese states constructed and maintained walls for centuries, and major surviving sections were built or rebuilt under dynasties including the Qin, Han, and Ming. The Ming-era walls near Beijing are among the best-preserved and most visited.

A 2009 survey estimated the total length of all known sections at 8,850 kilometres for the Ming wall alone, while a broader 2012 archaeological survey reported 21,196 kilometres for the entire historical system. The wall’s purpose included defense, border control, signaling, and regulation of movement. It is not reliably visible from the Moon with the unaided eye, a popular myth.