The Great wall of China

The Great Wall is perhaps China’s most famous and most mythologized site. Several sections are conveniently visited from Beijing, including at Badaling, the most popular site, about 70 km (43 mi.) northwest of Beijing and at Mutianyu, 90 km (56 mi.) northeast of Beijing. These impressive brick and earth structures date from the Ming dynasty, when the wall was fortified against Mongol forces to the north. The Ming wall is about 26 feet tall and 23 feet wide at the base, and could accommodate up to six horsemen riding abreast. Watch towers were built on high points every 200-300 meters or so with small garrison forces that could communicate with fire signals or fireworks. These stretches of the wall are part of a system that extends from the Shanhaiguan fortress on the Bohai Gulf in the east to the Jiayuguan fortress in the west, altogether some 6000 km (3700 mi).
The Ming sections of the wall are only a late stage in a long history, much of which has little to do with the present structures. The wall is most often associated with the First Emperor of China (Qin Shi Huangdi, reigned 221-210 BC), who after unifying China by conquest undertook to link up previously existing sections of walls belonging to conquered states, but on a course far to the north of the present wall. The First Emperor mobilized massive conscripted labor forces, including convicts and prisoners, by some accounts up to a million strong, to conduct this building campaign.