A series of fortification walls, constructed with stone, tamped earth, wood and any other locally available material, along east west line on the northern China were built against the raids and invasions of the various nomadic groups from the north. Several sections were built as early as 7th century BC and were later connected and strengthened. Especially famous is the wall built 220–206 BC by Qin Shi Huang, the first Emperor of China. Little of that wall remains. Since then, the Great Wall has on and off been rebuilt, maintained, and enhanced. The Ming Dynasty in the 14th century revived the wall and constructed stronger walls built with bricks and stones instead of tamped earth. The wall was enhanced with watch towers, troop barracks, and garrison stations. The wall included signaling capabilities through the means of smoke or fire and was also used as a major transportation corridor. The Great Wall is a World Heritage Site and is estimated to be 13,171 miles long including all it’s branches.