THE GREAT WALL
By Stella on 2016-08-23

The Great Wall of China is a series of stone and earthen fortifications in northern China, built originally    to protect the northern borders of the Chinese Empire against intrusions by various nomadic groups. Several walls have been built since the 5th century BC that are referred to collectively as the Great Wall, which has been rebuilt and maintained from the 5th century BC through the 16th century. One of the most famous is   the wall built between 220–206 BC by the first Emperor of China, Qin Shi Huang. Little of that wall remains;     the majority of the existing wall was built during the  Ming Dynasty. 

The Great Wall stretches from Shanhaiguan in the east, to Lop Nur in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. The most comprehensive archaeological survey has recently concluded that the entire Great Wall, with all of its branches, stretches for 8,851.8 km. This is made up of 6,259.6 km sections of actual wall, 359.7 km of trenches and 2,232.5 km of natural defensive barriers such as hills and rivers.

The Chinese were already familiar with the techniques of wall-building by the time of the Spring and Autumn Period. During the Warring States Period from the 5th century BC to 221 BC, the states of Qin, Wei, Zhao, Qi and Yan all constructed extensive fortifications to defend their own borders. Qin Shi Huang conquered all opposing states and unified China in 221 BC, establishing the Qin Dynasty. To protect the empire against intrusions by the Xiongnu people from the north, he ordered the building of a new wall to connect the remaining fortifications along the empire's new northern frontier. Most of the ancient walls have eroded away over the centuries, and very few sections remain today. 

 Later, the Han, Sui and Jin dynasties all repaired, rebuilt, or expanded sections of the Great Wall at great cost to defend themselves against northern invaders. Towards the end of the Ming Dynasty, the Great Wall helped defend the empire against the Manchu invasions. However, when the gates at Shanhaiguan were opened by Wu Sangui, the Manchus quickly seized Beijing and established the Qing Dynasty. Under Qing rule, China's borders extended beyond the walls and Mongolia was annexed into the empire, so construction and repairs on the Great Wall were discontinued.

（inrtoduction taken from the internet）