Defending the river and making use of the fortress city is a fortification idea put forward by Song Dynasty, which is backward in cavalry, to defend against the attack of northern minority troops who are good at bending horses. The reason is that "riding step by step is not good at taking risks." That is to say, it advocates "taking the ground array (taking the mountain spring as the array) to set risks and taking the mountain spring as the solid" (Song History, volume 273, Biography of He Cheng Moment). Accordingly, in the northern song dynasty, large and small lakes were dredged and diked to store water in Hebei border area to limit Liao soldiers; Along the borders of Hedong and Shaanxi, castles and villages will be built, and trenches will be dug to prevent Xia Bing. In the early Southern Song Dynasty, Song Ting did not take advantage of the danger of Jianghuai, but relied on important counties along the Yangtze River to organize a divisional defense system to prevent the attack of the nomads from attacking. At the end of the Southern Song Dynasty, Yu Jie, the general in charge of the Sichuan military, took an active part in guarding the city fortress and launched a large-scale city defense construction to prevent the Mongolian army from using troops in Sichuan. More than 0/0 cities such as Fishing City (now Hechuan East, Sichuan), Huo Da (now Cangxi South, Sichuan) and Qingju (now South, Sichuan) have been built successively. To build a walled city, "it's all based on mountains, chess and stars, governing the county, and gathering troops to keep the plan" (History of Song Dynasty, Volume 4 16, Yu Jie biography).