Introduction to the Fortress: The Fortress was the core of the former military defense line. The foundation stone was 3.7-meter-thick granite. The city wall was made of stone mixed with shell powder. It was 90 meters wide and 9.15 meters high. It was narrowed upward to 2.7 meters. meters thick, the entire fort covers an area of ​​about 10,000 square meters, with an irregular shape and a side length of about 100 meters.