Introduction to Seoul: It was named because the residents were Han nationality. It was built in the 9th year of Yongzheng in the Qing Dynasty (1731 AD). The "Green Camp Military City" was built for the army of General Ningyuan Yue Zhongqi. The city wall has a circumference of more than 4 kilometers and is rectangular, 1553.5 meters long from east to west and 788.7 meters wide from north to south. There is Wengcheng outside the west gate with a radius of 35.4 meters. It is basically preserved. The other three Wengcheng have been destroyed. Seoul has four city gates. All four gate towers on the city wall have been destroyed. There are also 3,600 crenellations, 7 forts, 8 horse paths, turrets at the four corners, a moat dug outside the city and 4 suspension bridges. The city wall is 6.8 meters high. , the bottom width is 6 meters, the top width is 4 meters, and the parapet built above is 0.5 meters high and 0.6 meters wide.