The Forbidden City in Beijing covers an area of 720,000 square meters, the size of 80 football fields, and houses a total of 980 buildings, making it the largest collection of preserved ancient wooden structures in the world, and the Hall of Supreme Harmony at the heart of the palace is the world's largest single building. The top photo shows the main north gate as seen from the hill Jing Shan (fig.), an artificial hill directly north of the Forbidden City, constructed from the soil excavated when digging the moats and nearby canals, while the picture below shows the northeastern corner tower, one of several watch towers, that –according to legend– are designed after the intricate cricket cage of one of the senior court eunuchs. The location of the hill is dictated by the rules of feng shui, which suggests a better flow of Qi for residences and tombs that are sited south of a nearby hill, thus channeling both harmful yin and cold northern winds away.