"Plate door" water and land gates Bingzhi imposing. Lu points inside and outside the double gate, inside and outside the city wall constitutes a fan about two 20-meter square Barbican. Ancient warfare to defend, to lure the enemy to the city, and from the city send the archers, falling stones, like "Wengzhongzhuobie" wipe out the invasion of the enemy, the left inside the north gate, and about 20 degrees of slope of the walls of Happy Valley Road, straight wall on top of a shop by the giant brick paved broad platform.