The Great Wall of China is a series of fortifications that were built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic groups from the Eurasian Steppe.
The Great Wall actually consists of numerous walls—many of them parallel to each other—built over some two millennia across northern China and southern Mongolia.
The most extensive and best-preserved version of the wall dates from the Ming dynasty (1368–1644) and runs for some 5,500 miles (8,850 km) east to west from Mount Hu near Dandong, southeastern Liaoning province, to Jiayu Pass west of Jiuquan, northwestern Gansu province.
The wall was originally conceived by Emperor Qin Shi Huang in the third century B.C.
Source: Britannica and Wikipedia