The Great Wall of China is a series of fortifications made of stone, brick, compressed earth, wood and other materials, generally built along an east-west line through China's historic northern borders to protect Chinese states and empires against raids and the invasions of the various nomadic groups of the Eurasian steppe. Several walls were built as early as the 7th century BC these, later joined together and made larger and stronger, are collectively referred to as the Great Wall. Especially famous is the wall built in 220 and 206 BC, by Qin Shi Huang, the first emperor of China. Little of that wall remains. The Great Wall has been rebuilt, maintained and improved on various dynasties; most of the existing wall comes from the Ming dynasty (1368 and 1644). In addition to defense, other Great Wall goals have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or the encouragement of trade and the control of immigration and emigration. In addition, the defensive features of the Great Wall were reinforced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through smoke or fire, and the fact that the Great Wall route also served as a transport corridor. The Great Wall extends from Dandong to the east to Lop Lake to the west, along an arch that roughly delineates the southern border of Inner Mongolia. The Chinese were already familiar with the construction techniques of the walls at the time of the spring and autumn period between the eighth and fifth centuries BC. During this period and the subsequent Warring States period, the states of Qin, Wei, Zhao, Qi, Yan and Zhongshan built extensive fortifications to defend their borders. Built to withstand the attack of small arms such as swords and spears, these walls were mostly made by molding earth and gravel between the frames of the boards. King Zheng of Qin conquered the last of his opponents and unified China as the first emperor of the Qin dynasty (Qin Shi Huang) in 221 BC. With the intention of imposing a centralized government and preventing the resurgence of the feudal lords, he ordered the destruction of the sections of the walls that divided his empire among the previous states. To place the empire against the Xiongnu people from the north, however, he ordered the construction of new walls to link the remaining fortifications along the northern frontier of the empire. Transporting the large amount of materials needed for construction was difficult, so builders have always tried to use local resources. The stones of the mountains were used on mountain ranges, while the clay was used for construction in the plains. There are no surviving historical documents indicating the exact length and course of the Qin walls. Most of the ancient walls have been eroded over the centuries and today very few sections remain. The human cost of construction is unknown, but it has been estimated by some authors that hundreds of thousands, if not up to a million workers have died building the Qin wall. Later, the Han, Sui, and Northern dynasties began to repair, rebuild, or expand sections of the Great Wall at a high price to defend themselves from the invaders of the north. The Tang and Song dynasties did not undertake any significant effort in the region. The Liao, Jin and Yuan dynasties, which ruled northern China for most of the X and XIII century, built defensive walls in the twelfth century, but those were located far north of the Great Wall as we know it, within the Chinese province of Inner Mongolia and in Mongolia itself.