The urban area of Beijing, on the plains in the south-central of the municipality with elevation of , occupies a relatively small but expanding portion of the municipality's area. The city spreads out in concentric ring roads.  The Second Ring Road traces the old city walls and the Sixth Ring Road connects satellite towns in the surrounding suburbs. Tian'anmen and Tian'anmen Square are at the center of Beijing, directly to the south of the Forbidden City, the former residence of the emperors of China. To the west of Tian'anmen is Zhongnanhai, the residence of China's current leaders. Chang'an Avenue, which cuts between Tiananmen and the Square, forms the city's main east–west axis.