About Beijing 
The Great Wall of China is the longest structure ever built. It is about 4,000 miles long, and it was built entirely by hand. About 1, 200 - 1, 500 miles of the Great Wall were built during the reign of Emperor Shi Huangdi (Qin Dynasty). The Great Wall crosses northern China from the east coast to the central part of China.
The genesis of the Great Wall dates to the Warring Sttes period from 475 to 221 BC, when Chinese feudal kingdoms built earthen ramparts to defend against nomadic invaders. It was under the fierce emperor Qin Shihuang, who unified China in 221 BC that the Great Wall really began to take shape. He conscripted some 300,000 laborers to work for ten years on joining the various pre-existing sections into a single fortified wall. The suffering of the workers who toiled in freezing winters and scorching summers became legendary. As dynasty came into being and passed into history, the Great Wall was continuously repaired and extended.
The sections of the Great Wall near Beijing were renovated during the Ming dynasty, whose leaders spent a century strengthening and extending the Wall to the Yellow Sea. The previous ramparts, which were made of stones , packed earth and wood, were covered by Ming builders with bricks. They built crenellations to protect archers, widened the Wall so it could accommodate five horses abreast, and added many watchtowers. A system of beacons lit from tower to tower ensured that enemy troop movements were swiftly relayed to headquarters.
Despite such defensive features, the Great Wall failed in its purpose of keeping out invaders. It was breached several times, notably by the armies of Genghis Khan in 1215 and by Manchu troops in 1644. Conversely, the Great Wall was a tremendous success in forging a sense of nationhood since it marked the physical boundary between China and abroad, and the psychological boundary between civilization and chaos. Scaling forbidding landscapes of mountains and deserts, the Great Wall was also a triumph of the emperor's will over nature.
