Named as ‘Treasure Gate’ in ancient times, it was the southern city-gate of Nanjing as the capital of the Ming Dynasty. It is marked by strict and regular layout of a unique structure, which is wrapped with iron on both sides with a groove cut inside for fastening the gate with a bolt. In the era of cold weapons, with the gate and the sluice closed right after enemy troops stormed into the city, they would be trapped in the enclosure. Inside the castle, there are 27 caves capable of hiding over 3,000 soldiers.