First, when the city wall was built in the Ming Dynasty, it may be that other watchtowers were seriously damaged at that time, only the southwest watchtower was not seriously damaged, so in order to save labor and construction costs, the rounded corners of the southwest watchtower were used in the Tang Dynasty without reconstruction.
Second, when planning the Xi 'an City Wall, the ancient Tang Cheng site was accidentally discovered in the southwest corner. The site is semi-circular, so it is convenient to use the original circular foundation in order to save time and effort.
Thirdly, the traditional architecture in China is influenced by the requirements of the Eight Diagrams and Feng Shui, and the architecture cannot be too symmetrical, so it is an "asymmetric" architecture.
Statement 4: The southwest corner of the city is located in the ground fissure zone, and there is a ground fissure near the southwest corner of the city wall and parallel to the city wall. As long as the corner platform is square, there will be problems once it is repaired, and finally it will be impossible to keep it round.
Of the four corners of Xi 'an city wall, why is only the southwest corner round and the other three square? In fact, if this issue is not mentioned, some local citizens have not noticed the difference between the southwest corner and the other three places. There are different opinions about why only the southwest corner is round.
There are also many folk stories, whether they are fabricated or legendary. The circle in the southwest corner of Xi 'an city wall has been standing there. So how do you understand that only the southwest corner is round?