The convex shape of Beijing was formed in Jiajing period of Ming Dynasty. During the Jiajing period, a number of ritual and music buildings were built and rebuilt, and the walls of Beijing City were added. Finally, the urban outline of Beijing changed from the original mouth shape to a convex shape, forming an urban pattern of inner city and outer city.
In the thirty-second year of Jiajing in Ming Dynasty (1553), Sejong decided to build the outer city of Beijing in order to resist the invasion of Mongolian army. However, only one wall in the south was completed, and the finance turned red. After consulting with ministers, we can only adopt a compromise, that is, building two north-south walls connecting the inner and outer cities. So Beijing changed from the original mouth shape to a convex shape.
Extended data:
Architectural changes in Beijing during the Ming and Qing Dynasties
The Ming Imperial City is outside Miyagi, with about six gates at 18. Due south is Daming Gate (changed from Qing Dynasty to Qing Dynasty), which turns east to Chang 'an Left Gate, west to Chang 'an Right Gate, east to Dong 'anmen, west to Xi 'anmen and north to Bei 'anmen (changed from Qing Dynasty to Di 'anmen). On both sides of the East and West Courtyards of Chengtianmen, there are ancestral temples and social altars of "Zuo Zu You She".
This arrangement changed the layout of "Zuo Zu You She" far away from the imperial city in Yuan Dynasty, and made the ancestral hall and social altar closely connected with the palace. There is a broad imperial road in front of Chengtianmen, and a thousand-step promenade connects the willows and ridges on both sides to form a T-shaped square. Outside the palace walls on both sides of the square, the main offices of the central government are symmetrically arranged.
Thus changed the scattered layout of the central government agencies in the metropolitan area of the Yuan Dynasty. In this way, the central yamen in front of the palace, the "Zuo Zu You She", the imperial garden behind the palace, the imperial tomb in the left of the palace and the Taiye Pool in the right of the palace are all surrounded by the imperial city.
Introduction of Beijing City in Qing Dynasty: The inner and outer cities, the imperial city and the Forbidden City in Ming Dynasty were inherited in Qing Dynasty, and their positions remained unchanged. It's just that the name of Guangning Gate in the outer city is about Ning, and the word "Ning" is taboo, so Guangning Gate was changed to Guang 'anmen. The inner city is divided into nine gates, Zhengyang Gate is in the south center, Chongwen Gate is on the left and Xuanwu Gate is on the right.
The east gate is Chaoyangmen and Dongzhimen; The two gates in the west of the city are Fuchengmen and Xizhimen respectively; To the north are Deshengmen and Andingmen. The outer city is divided into seven gates, namely Yongdingmen in the south, Zuo 'anmen and You 'anmen on both sides, Guang 'anmen in the west and Guangqumen in the east. At the junction with the inner city, there are East Gate and West Gate.
Baidu encyclopedia-Beijing city