In the depths of Yanmen Mountain in Shanxi, there stands a lonely frontier outpost, Yanmenguan, some 2,000 years old.
The pass consists of a pass city, a barbican, and an enclosing wall. The pass city is located between two rolling hills, with a height of 30 feet (nine meters) and a perimeter of 1,100 miles (1,006 meters). The enclosing wall stretches outwards from the east and west wings of the pass city, measuring more than three miles (five kilometers) long, and it is dotted with beacon towers. The barbican was built outside the north gate of the pass city.
