<P> 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 . Intending to impose centralized rule and prevent the resurgence of feudal lords, he ordered the destruction of the sections of the walls that divided his empire among the former states . To position the empire against the Xiongnu people from the north, however, he ordered the building of new walls to connect the remaining fortifications along the empire's northern frontier . "Build and move on" was a central guiding principle in constructing the wall, implying that the Chinese were not erecting a permanently fixed border . Transporting the large quantity of materials required for construction was difficult, so builders always tried to use local resources . Stones from the mountains were used over mountain ranges, while rammed earth was used for construction in the plains . There are no surviving historical records indicating the exact length and course of the Qin walls . Most of the ancient walls have eroded away over the centuries, and very few sections remain today . The human cost of the construction is unknown, but it has been estimated by some authors that hundreds of thousands, if not up to a million, workers died building the Qin wall . Later, the Han, the Sui, and the Northern dynasties all repaired, rebuilt, or expanded sections of the Great Wall at great cost to defend themselves against northern invaders . The Tang and Song dynasties did not undertake any significant effort in the region . The Liao, Jin, and Yuan dynasties, who ruled Northern China throughout most of the 10th--13th centuries, constructed defensive walls in the 12th century but those were located much to the north of the Great Wall as we know it, within China's province of Inner Mongolia and in Mongolia itself . </P> <P> The Great Wall concept was revived again under the Ming in the 14th century, and following the Ming army's defeat by the Oirats in the Battle of Tumu . The Ming had failed to gain a clear upper hand over the Mongolian tribes after successive battles, and the long - drawn conflict was taking a toll on the empire . The Ming adopted a new strategy to keep the nomadic tribes out by constructing walls along the northern border of China . Acknowledging the Mongol control established in the Ordos Desert, the wall followed the desert's southern edge instead of incorporating the bend of the Yellow River . </P> <P> Unlike the earlier fortifications, the Ming construction was stronger and more elaborate due to the use of bricks and stone instead of rammed earth . Up to 25,000 watchtowers are estimated to have been constructed on the wall . As Mongol raids continued periodically over the years, the Ming devoted considerable resources to repair and reinforce the walls . Sections near the Ming capital of Beijing were especially strong . Qi Jiguang between 1567 and 1570 also repaired and reinforced the wall, faced sections of the ram - earth wall with bricks and constructed 1,200 watchtowers from Shanhaiguan Pass to Changping to warn of approaching Mongol raiders . During the 1440s--1460s, the Ming also built a so - called "Liaodong Wall". Similar in function to the Great Wall (whose extension, in a sense, it was), but more basic in construction, the Liaodong Wall enclosed the agricultural heartland of the Liaodong province, protecting it against potential incursions by Jurched - Mongol Oriyanghan from the northwest and the Jianzhou Jurchens from the north . While stones and tiles were used in some parts of the Liaodong Wall, most of it was in fact simply an earth dike with moats on both sides . </P> <P> Towards the end of the Ming, the Great Wall helped defend the empire against the Manchu invasions that began around 1600 . Even after the loss of all of Liaodong, the Ming army held the heavily fortified Shanhai Pass, preventing the Manchus from conquering the Chinese heartland . The Manchus were finally able to cross the Great Wall in 1644, after Beijing had already fallen to Li Zicheng's rebels . Before this time, the Manchus had crossed the Great Wall multiple times to raid, but this time it was for conquest . The gates at Shanhai Pass were opened on May 25 by the commanding Ming general, Wu Sangui, who formed an alliance with the Manchus, hoping to use the Manchus to expel the rebels from Beijing . The Manchus quickly seized Beijing, and eventually defeated both the rebel - founded Shun dynasty and the remaining Ming resistance, establishing the Qing dynasty rule over all of China . </P>

About the great wall of china in english