Beijing is a metropolis in northern China and the capital of the People's Republic of China. It is one of the four municipalities of the PRC, which are equivalent to provinces in China's administrative structure. Beijing is one of the Four Great Ancient Capitals of China. At the heart of Beijing's historical center lies the Forbidden City, the enormous palace compound that was the home of the emperors of the Ming and Qing dynasties; the Forbidden City also hosts the Palace Museum, which contains imperial collections of Chinese art. Surrounding the Forbidden City are several former imperial gardens and parks, notably the Beihai, Houhai, Shichahai, Zhongnanhai, Jingshan and Zhongshan, all of which are accessible for tourists except Zhongnanhai, the headquarters of the Communist Party of China. From the Tiananmen Square, which is located right across the Forbidden City, there are several notable sites, such as the Tiananmen, the Great Hall of the People, National Museum of China, Monument to the People's Heroes, and Mausoleum of Mao Zedong. The Summer Palace and the Old Summer Palace both lie at the western end of the urban city of Beijing; the Summer Palace, a UNESCO World Heritage Site, displays a comprehensive collection of imperial gardens and palaces that functioned as the summer retreat for the Qing Dynasty emperors.