Introduction to the south wall of the outer city: The south wall of the outer city is the main existing relic of the ancient castle. It is more than three feet high and about 30 meters long. It is made of bricks made of a mixture of glutinous rice, molasses, and oyster shell ash. It is very strong. Nowadays, ancient banyan roots are entrenched here, full of ancient charm, and it has been planned as an ancient wall historical park.