The Tujue, who built a great empire that was able to mobilize 400,000 soldiers and to occupy territories by 10,000 li(里) from the late 7th century to the early 8th century, went beyond the Yellow River and turned the vast Ordos borderlands into a battlefield by invasion and plunder. The Tang dynasty came to fall in an emergency in the early 8th century when the Tujue won a victory at the massive battle of Mingsha district(鳴砂縣), south of the Ordos, and then advanced as far as the Tang’s capital city, Chang’an. The Tang dynasty had to seek for a fundamental measure to face against the Tujue’s threat, which was the Sanshouxiangcheng(three cities for reciving surreders) that was built at the suggestion of Zhang Renyuan.
The Sanshouxiangcheng was composed of three fortresses built in AD 708 on the north side of the Yellow River, north of the Ordos. The Yellow River formed the area like the Chinese letter 几(ji) where the Ordos region was located inside. The Sanshouxiangcheng was built along the northern line of the 几(ji) with Xishouxiangcheng(西受降城) at the western end, Dongshouxiangcheng(東受降城) at the eastern end, and Zongshouxiangcheng(中受降城) at the midpoint. The distance between each fortress was 400 li, and so 800 li in total along the Yellow River from east to west. The defence range of the Sanshouxiangcheng covered not only the east and west, but also north of Niutouchonashan(牛头朝那山) located about 300 li far from it. The way of controlling the region was as follows: first, some 1,800 beacon stations were installed in those areas, east, west, north and south; second, military bases and installations including the three fortresses and the 1,800 beacon stations established military network shaped like spiderweb; third, a series of institutions were prepared, such as military deployment, transportation and supplies, that enabled those fixed military installations to function efficiently.
The Sanshouxiangcheng was a borderline, artificially constructed on the northern side of the Yellow River, a natural defence line, to separate the agricultural Tang from the nomadic Tujue. The Sanshouxiangcheng had a self-defence nature for the Tang because it was built to defend against the repeated invasions of the Tujue. On the other hand, the fact that the Sanshouxiangcheng was built in the territory of the Tujue beyond the Yellow River, the Tang’s border at the time, also made the aggression of the Tang undeniable. Nevertheless, the Sanshouxiangcheng was a landmark for Han Chinese writers in the mid and late Tang, Song, Ming dynasties when the northern nomadic threat of invasion towards the Han Chinese dynasties was great. Thus, it was presented as a typical example of considering the Sanshouxiangcheng to emphasize the nature of self-defence against the barbaric Tujue and to defend against the invasion of the northern peoples.