The Forbidden City, rectangular in shape is the world's largest palace complex and covers 720,000 square meters (178 acres, or 0.28 square miles). Surrounded by a six meter deep moat and a ten meter high wall are five halls, seventeen palaces, and numerous other buildings. The Forbidden Palace is reputed to have a total of 9,999.5 rooms. However, according to surveying by the Palace Museum, there are about 8,600 existing rooms. (The majority of buildings in the Forbidden City have an odd number of rooms, distributed symmetrically about an axis. However, the Imperial Library had six rooms as a charm against fire, because the number six is associated with water in astrology. So that the building does not look out of place, the sixth room was built very small, hence the half-room.)