BEIJING
Beijing is situated at the northern tip of the roughly triangular North China Plain, which opens to the south and east of the city. Mountains to the north, northwest and west shield the city and northern China's agricultural heartland from the encroaching desert steppes. The northwestern part of the municipality, especially Yanqing County and Huairou District, are dominated by the Jundu Mountains, while the western part is framed by Xishan or the Western Hills. The Great Wall of China across the northern part of Beijing Municipality was built on the rugged topography to defend against nomadic incursions from the steppes. Mount Dongling, in the Western Hills and on the border with Hebei, is the municipality's highest point, with an altitude of 2,303 metres (7,556 ft).
Major rivers flowing through the municipality, including the Chaobai, Yongding, Juma, are all tributaries in the Hai River system, and flow in a southeasterly direction. The Miyun Reservoir, on the upper reaches of the Chaobai River, is the largest reservoir within the municipality. Beijing is also the northern terminus of the Grand Canal to Hangzhou, which was built over 1,400 years ago as a transportation route, and the South–North Water Transfer Project, constructed in the past decade to bring water from the Yangtze River basin.
The urban area of Beijing, on the plains in the south-central of the municipality with elevation of 40–60m, 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.
THE GREAT WALL OF CHINA
The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe. Several walls were being built as early as the 7th century bc; these, later joined together and made bigger and stronger, are now collectively referred to as the Great Wall. Especially famous is the wall built 220–206 bc by Qin Shihuang, the first Emperor of China. Little of that wall remains. Since then, the Great Wall has on and off been rebuilt, maintained, and enhanced; the majority of the existing wall is from the Ming Dynasty.
Other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor.
The main Great Wall line stretches from Shanhaiguan in the east, to Lop Lake in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. A comprehensive archaeological survey, using advanced technologies, has concluded that the Ming walls measure 8,850 km (5,500 mi). This is made up of 6,259 km (3,889 mi) sections of actual wall, 359 km (223 mi) of trenches and 2,232 km (1,387 mi) of natural defensive barriers such as hills and rivers. Another archaeological survey found that the entire wall with all of its branches measure out to be 21,196 km (13,171 mi).
