The entrance to the fortress consists of 4 gates and 3 inner chambers. Using this layout, enemies can be stopped at each gate and taken out. Those who make it past the first gate are then trapped in the inner chambers for the guarding soldiers to fight. By the time the fortress is breached, many opposing soldiers would have been killed. The Japanese learned this. And when they finally broke through and took over Nanjing, the conducted the Nanjing Massacre.