The Great Wall of China is a Chinese fortification built from 3rd century BC until the beginning of the 17th century, in order to protect the various dynasties from raids by Mongol, Turkic, and other nomadic tribes coming from areas in modern-day Mongolia and Manchuria. The Wall has been periodically rebuilt and modified throughout history by each reigning Chinese dynasty. There have been five major walls: 208 BC (Qin Dynasty), 1st century BC (Han Dynasty), 7th century CE (Sui Dynasty), 1138-1198 (Five Dynasties and Ten Kingdoms Period) and 1368-1640 (from Hongwu Emperor until Wanli Emperor of the Ming Dynasty). Today the great wall stretches over a formidable 6,352 km, from Shanhai Pass on the Bohai Sea in the east, at the limit between "China proper" and Manchuria, to Lop Nur in the southeastern portion of Xinjiang Uygur Autonomous Region. Ancient records report that at least one million slaves and prisoners of war were used to create this defensive wall. Many laborers died from exhaustion and starvation while working on this colossal task, their bodies added to the rubble and masonry as the quickest means of disposal. For centuries, the Wall was known as "the longest cemetery in the world." The Great Wall which tourists visit today was built during the Ming Dynasty, starting around the year 1368 and lasting till around 1640. Work on the wall started as soon as the Ming took control of China but initially, walls were not the Ming's preferred response to raids out of the north. That attitude began to change in response to the Ming's inablilty to defeat the Oirat war leader Esen Taiji in the period 1449 to 1454. A huge Ming Dynasty army with the Zhengtong Emperor at its head was annihilated in battle and the Emperor himself held hostage in 1449. The real focus on wall building started as a result of Altan Khan's siege of Beijing in 1550. The Ming, faced with the choice of trying to defeat the Mongols with direct military force, chose instead to build a massive defensive barrier to protect China. As a result, most of the Ming Great Wall was built in the period 1560 to 1640. This new wall was built on a grand scale with longer lasting materials (solid stone used for the sides and the top of the Wall) than any wall built before.