The origins of the Great Wall date back to the 7th century BC, when fortifications and walls were constructed as a defense from the invasions of northern nomadic tribes. After the unification of the country in 221 BC by the Emperor Qin Shi Huang, in the Qin dynasty, these walls were connected and extended to form a defensive system on the northern border. It was called Wan Li Chang Cheng or 10.000 Li Great Wall, with a length of more than 5.000 km (10.000 Li). Rebuilding and further extensions were made mainly in the Han (202 BC - AD 220) and Ming (1368 - 1644) dynasties.