Yin Mountains
For the most part, barring intermittent passes and valleys (the major one being the corridor through Zhangjiakou and the Juyong Pass), the North China Plain remained shielded from the Mongolian steppe by the Yin Mountains. However, if this defence were breached, China's flat terrain offered no protection to the cities on the plain, including the imperial capitals of Beijing, Kaifeng, and Luoyang. Heading west along the Yin Mountains, the range ends where the Yellow River circles northwards upstream in the area known as the Ordos Looptechnically part of the steppe, but capable of irrigated agriculture. Although the Yellow River formed a theoretical natural boundary with the north, such a border so far into the steppe was difficult to maintain. The lands south of the Yellow River—the Hetao, the Ordos Desert, and the Loess Plateau—provided no natural barriers on the approach to the Wei River valley, the oft-called cradle of Chinese civilization where the ancient capital Xi'an lay. As such, control of the Ordos remained extremely important for the rulers of China: not only for potential influence over the steppe, but also for the security of China proper. The region's strategic importance combined with its untenability led many dynasties to place their first walls here.