The Great Wall of China is one of the most famous structures on Earth, but it…

The Great Wall of China is one of the most famous structures on Earth, but it is also one of the most misunderstood. To begin with, it is not a single wall. It is a vast collection of walls, watchtowers and fortresses built by different Chinese states and dynasties over a period of roughly two thousand years.

The earliest walls were built as early as the seventh century BC, when rival states constructed defences against each other and against nomadic peoples from the north. After the first emperor, Qin Shi Huang, united China in 221 BC, he ordered many of these walls to be connected and extended. Later dynasties continued to build, repair and abandon sections according to their needs.

Most of the sections that tourists visit today, with their neat brick walls and watchtowers, date from the Ming dynasty, which ruled from 1368 to 1644. Earlier walls were often made of rammed earth, pressed layer by layer between wooden boards, or of local stone. Ming builders used bricks on a much larger scale, and researchers have found that some of their mortar contained sticky rice, which helped to make it remarkably strong and durable.

How long is the Great Wall? The answer depends on what is counted. A survey published by Chinese authorities in 2012, which included all known sections from different periods, gave a total length of more than 21,000 kilometres. However, many of these sections are now in ruins or have disappeared almost completely.

One of the most popular myths about the wall is that it can be seen from space with the naked eye, or even from the Moon. This is not true. Although the wall is extremely long, it is relatively narrow and similar in colour to the surrounding landscape. Astronauts, including China's first astronaut, Yang Liwei, have reported that they could not see it from orbit without help.

Despite its size, the wall was not always effective as a defence. Invaders sometimes simply went around it, or bribed the guards. But it served other purposes too: watchtowers allowed signals to be passed quickly using smoke or fire, and the wall helped to control trade and movement along the frontier. Today it is a UNESCO World Heritage Site and one of China's most visited attractions, although parts of it are threatened by erosion and by tourists taking bricks as souvenirs.