The Great Wall of China is a huge fortification in northern China. It is more than 3,900 miles long. Work on the wall continued throughout Chinese history, from 220 B.C. (Qin Dynasty) to 1640 AD (Ming Dynasty). It is often referred to as "The Longest Cemetery on Earth" because thousands of people died during its construction. The Great Wall was built for three reasons: 1.) To keep invading Mongols out of China; 2.) To united the Warring States into one Chinese empire; 3.) To show the power of the Emperors. Construction on the wall ceased in 1644 when China was conquered by the Manchu Dynasty.