Simatai, located in the north of Miyun County 120 km from Beijing, holds access to Gubeikou, a strategic pass in the eastern part of the Great Wall. It was originally built during the Northern Qi Dynasty (550-577) and rebuilt during the Wanli reign of the Ming Dynasty (1368-1644). This section of the Great Wall hangs precariously onto the Yanshan Mountains. Its structure is unique in that it contains single and double walls and assumes a trapezoidal shape. 
The watchtowers are round or oblate in shape and composed of two or three floors. Their roofs are varied as well-some are flat, some look like the awning of a boat, and the others are domed. The Simatai section of the Great Wall stands out as the most fascinating example of architecture along the entire Great Wall.
Thrilling are the sights of Simatai, which is poised on the brow of a razor sharp cliff, and narrows down to a mere 40 millimeters at a place known as "Heavenly Bridge". The "Sky Ladder" leans against a mountain slope with a 90-degree gradient.
Simatai is also known for its two major watchtowers. Wangjinglou, or "Beijing-Watching Tower", sits atop a peak nearly 1,000 meters above sea level, with lights in downtown Beijing shimmering faintly in the distance. The walls here are built of bricks stamped with the dates on which they were made and the code numbers of the armies who made them. Xiannulou, or Angel's Tower, is exquisite in structure and famed in fable and history.
