What was the Great Wall of China built for, and can you see it from space?

The Great Wall is not a single wall but a system of fortifications built, rebuilt, and extended across more than two millennia by successive dynasties, with the earliest sections dating to the Warring States period before 221 BCE and the sections tourists visit today mostly built during the Ming dynasty between the fourteenth and seventeenth centuries. Its primary purpose was defense against nomadic incursions from the northern steppe, but it functioned as much as an administrative instrument: controlling movement across the frontier, taxing trade along the routes, regulating immigration and emigration, and providing a communication system through beacon towers that could relay signals over long distances by smoke and fire.

How well it worked is debated. It never made the frontier impermeable, and both the Mongols under Genghis Khan and later the Manchus entered China despite it, the latter through a gate opened by a defecting general. The more defensible assessment is that it raised the cost and slowed the speed of raiding, channeled movement to defensible points, and served political and symbolic functions alongside military ones. Its construction cost enormous amounts of labor, much of it conscripted, with substantial loss of life.

The visible-from-space claim predates spaceflight and is false. The wall is long but only a few meters wide and built from local materials that blend with surrounding terrain, and at orbital altitude it is far below the resolution of unaided human vision. Astronauts including Chinese astronaut Yang Liwei have stated they could not see it. Under exceptional conditions it can be picked out in photographs from low orbit with magnification, as can highways and airports, which makes it unremarkable rather than unique. Total length across all branches and periods has been surveyed at over 21,000 kilometers.