The Palace Museum known as the Purple Forbidden City, was the imperial palace of the Ming and Qing Dynasties. It is the largest, best preserved palace complex remaining in the world today. In the feudal society emperors had supreme power, so this residence was certainly a forbidden place. Purple was the symbolic color of the North Star, so the palace was believed to be the center of the cosmos. The Forbidden City is located in the center of Beijing. It lies on the axis of Beijing City and faces south, with Coal Hill at its back, the Ancestra Temple in the front to the left and the Altar of Land and Grain in the front of the right. The Forbidden City is rectangular in shape, 961 meters long from north to south and 753 meters wide from east to west. The outer wall is 10 meters high. There are four watch towers in the corners. Outside the wall is the 52-meter-wide moat. The Forbidden City covers an area of 72 hectares and the total floor space is about 150,000 square meters. According to the calculation in 1973, there are 8,704 room-units inside the whole complex. Large-scale construction of the palace started in 1406. It took 14 years and was completed in 1420. The construction involved 230,000 artisans and one million civilians. In 1421, the capital of the Ming Dynasty was moved from Nanjing to Beijing. Since then 24 emperors resided here. Fourteen of them were Ming emperors and the other ten were Qing emperors.