Why is the southwest corner of Xi 'an a garden?

Traditionally, the city corners are square, but the southwest corner of Xi' an city wall is rounded. This rounded corner is the ruins of the turret of the "Imperial City" in the Tang Dynasty. The original site is round. When Ming City was built, the southwest corner was not rebuilt, and this unique corner was formed by adding bricks and soil as it was. A watchtower was built at each corner to assist the watchtower in wartime and defend the four sides. At this point, the high-altitude defense network on Xi' an city wall has formed a system, that is, the moon tower echoes the big tower from a distance to control the situation of suspension bridge and gate; Turret town guards the four corners of the city, assists the tower, and transmits the general's orders and the situation in the city; On the other hand, the watchtower uses its outstanding characteristics to form a three-way cross fire between the front and the two floors, attacking the city gate and reducing the pressure on the city gate. These affiliated military facilities complement each other with the city wall, forming a deep trench and high-rise military fortress, which really makes the defense system of Xi' an city wall "impregnable".