Zhonghua Gate is the main entry point along the historic Nanjing city wall today. The walls date from the start of the Ming Dynasty in the 14th century, and were the most extensive in the world at the time. This is not a simple door in the wall. It is a reinforced defense structure, cemented by mortar frm glutinous rice, with gates within gates and chambers that can confuse and trap the enemy.
These holes are dug deep into the wall, where hundreds of soldiers can be held. As the enemy breaches the wall and enters the pit, the waiting soldiers are released to surround, trap, and terminate the invaders. What a tactical design!
The original wall was 33km long, although much of it has survived.