History of the Great Wall of China the three, China's chief concern since the earliest times had been Mongolia – the home of many of the country's fiercest enemies including the Xiongnu, the Xianbei, the Khitans, and the Mongols. The Gobi Desert, which accounts for two-thirds of Mongolia's area, divided the main northern and southern grazing lands and pushed the pastoral nomads to the fringes of the steppe. On the southern side (Inner Mongolia), this pressure brought the nomads into contact with China. For the most part, barring intermittent passes and valleys (the major one being the corridor through Zhangjiakou and the Juyong Pass), the North China
Gobi Desert as a buffer against further desertification. The Gobi, broadly defined, can be divided into five distinct dry ecoregions, based on variations in climate and topography: The surface is extremely diversified, although there are no great differences in vertical elevation. Between Ulaanbaatar () and the small lake of Iren-dubasu-nor (), the surface is greatly eroded. Broad flat depressions and basins are separated by groups of flat-topped mountains of relatively low elevation ), through which archaic rocks crop out as crags and isolated rugged masses. The floors of the depressions lie mostly between above sea-level. Farther south, between Iren-dutiasu-nor and the Yellow