Great Wall of China, Jinshanling

Not one wall, and not one age

The Great Wall is not a single wall, and not from a single time. What most people picture (broad grey-brick ramparts marching over green mountains) is overwhelmingly the work of the Ming dynasty, built and rebuilt across the fifteenth and sixteenth centuries.

The first long frontier walls were joined more than 1,700 years earlier, under Qin Shi Huang around 220 BC, and were mostly rammed earth and rubble. The brick-and-stone wall the world visits at Badaling, Mutianyu and Jinshanling is Ming. Counting every dynasty's walls, surveys put the total length above 21,000 kilometres; the Ming wall alone runs roughly 8,850 kilometres.

A wall that could talk

The wall was less a barrier than a system. Its real power was communication.

Watchtowers and beacon platforms stood within sight of one another along the whole line. A threat spotted at one tower was passed by smoke columns in daylight and fire at night, the number of signals coding the size of the enemy force; a warning could travel hundreds of kilometres faster than any rider. Garrisons lived in and behind the wall, controlling the gates through which trade and people crossed the frontier.

The myth from space

The most repeated "fact" about the Great Wall, that it is the only human structure visible from space, or from the Moon, is false.

It is long but narrow, and roughly the colour of the land around it. Astronauts have confirmed it cannot be picked out with the naked eye from low orbit, let alone the Moon. The wall's true scale is horizontal, not visual: a line of stone and earth drawn across an entire subcontinent's northern edge.