Moon City, also known as urn city or sub City, is a small city built outside the city gate to shield the city gate, with the purpose of enhancing the defense of the city. According to Shoucheng, the first collection of the general principles of Wujing, "the urn outside the city is round or square, depending on the terrain. Gao Hou and the city have only one door to open, and the left and the right are as they please. "

In 1372, when Qiu Guang, commander of the guard of Changsha Government, rebuilt the earth city wall with bricks and stones, no moon city was built outside the gate of nine cities. According to the general annals of Hunan in the Qing Dynasty, "in the 11th year of Chongzhen (1638), the magistrate of Changsha was promoted to build the moon city". In the 25th year of Jiaqing (1820), the magistrate of Changsha and Shanhua asked for money to repair the city wall. At the same time, the Tianxin pavilion was expanded into three floors, and your city wall was built into two inner and outer cities, and the outer city was built into two North and South moon cities. In the third year of tongzhi (1864), governor Yun Shilin rebuilt the moon city and the city wall to meet the need of expanding the Tianxin Pavilion.

As for the specific use of the moon city, the guide to Changsha City written by Zou yubai in 1933 states: "the walls beside the pavilion form two huge caves, one on the left and one on the right. In the past, there were also places to guard the city and collect soldiers." The moon city has an area of about 500 square meters, which can hold hundreds of soldiers

1. When fighting, soldiers can take turns to mount the city, and the next soldiers can go down to the moon city to get a timely rest, which is relatively safe and can protect the troops. At the same time, it is also convenient to quickly replace the pre battle reduction.

2. There are eight big gun holes on the wall of Yuecheng city. The guns are hidden in the holes, which can avoid the bombardment of enemy guns, and the soldiers are relatively safe.

3. In general, the moon city is built outside the gate to protect the gate. The Garrison has an additional fortification, which increases the difficulty of enemy siege. However, Tianxin doesn't have a gate. No matter how hard the enemy attacks, it will not be allowed to enter. For the enemy who doesn't understand Tianxin you and has no city gate, it's like entering a maze array by mistake. He can't attack all the time. In this way, the garrison can easily control the enemy forces and increase their casualties. When the Taiping army attacked Changsha in 1852, they saw that the only tower in Changsha City was Tianxin Pavilion. They mistakenly thought that there must be a gate below, so they took Tianxin Pavilion as one of the main targets. They spent a lot of time and forces in vain. This may be the confusion of your heavenly heart's moon city.