4. Great Wall of China

Great Wall of China or the Great Wall is the longest structure ever built by humans, is located in the People's Republic of China. Its length is 6400 kilometers (from the area Sanhai Pass in the east to Lop Nur in the west) and the height of 8 meters in order to prevent the incursion of the Mongols from the north at that time. The width of the top 5 m, while the bottom width of 8 m. Every 180-270 m made a kind of watchtower. High watchtower is 11-12 m. To make this giant wall, it could take hundreds of years at the time of the various emperors.