Why are the two north corners of the inner wall of Beijing not right angles?

For a long time, people have been unable to solve this mystery. Some people analyze the terrain: the northern wall of the Yuan Dynasty is five miles north of Deshengmen and Andingmen, and the remains still exist. Its northwest corner is not abnormal, it is a right angle. In the Ming Dynasty, Beijing was rebuilt. For defense, the north of the city was abandoned and a new city wall was built five miles south of the original city wall. The western section of the newly-built North City Wall passes through the narrowest part of the Jishuitan, and then turns to the southwest, separating the western end of the Jishuitan from the city, so the northwest corner becomes an oblique angle. In the early Ming Dynasty, the water in Jishuitan was much deeper and the area was much larger than it is now. In order to strengthen the city wall and meet the needs of the building, it is reasonable to chamfer the city wall according to the terrain, so this view is accepted by many people.

The second view is that from the analysis of foreign satellite images, the northwest corner of Beijing has both right-angled and inclined wall-based images. The included angle between the two walls is 35 ~ 36, and the baseline of the east and west walls is located near the north bank of the northwest end of Haizi in Yuan Dynasty, which is at the same latitude as the east wall, indicating that the city wall was indeed built here. But why didn't it finish? It can also be seen from satellite images that there is a stratigraphic fault zone from Chegongzhuang to Dewai Street, which passes through the northwest corner of the city and intersects with that right-angled side obliquely. The present Beijing city was built in Yongle period of Ming Dynasty. When completed, the four corners of Beijing are right angles. However, during the Ming and Qing Dynasties, strong earthquakes often occurred in Beijing and its vicinity. Every earthquake, the wall from Xizhimen to Xinjiekou in the northwest corner of Beijing will collapse. Although it has been rebuilt many times, it is always collapsed by earthquakes. According to research, the original underground foundation is not firm, and there may be active faults. Your Majesty had to yield to the earthquake and decided to shrink the wall in the northwest corner to avoid unstable areas. Beijing has experienced several earthquakes, and the city walls have never collapsed. That's why a corner is missing.