Jiayuguan Pass Fortress - China

What we call the Great Wall of China was not initially built as a single, continuous wall. It is actually a set of fortifications and walls, built in sections. Jiayuguan is the closest section to Beijing and home to one of the wall's largest forts. Genghis Khan even once led his troops through this fort during his conquest of Chinese territory.

CN-3546015, sent by Annie.

Jaiyuguan (Jiayu Pass) is the most western end of the Great Wall. Also known as "the First and Greatest Pass under Heaven", Jiayu Pass is the best preserved and most magnificent military building among the passes of the Great Wall.

It is one of the main passes of the Great Wall, along with Juyong Pass and Shanhai Pass. It is located at the narrowest point of the western part of Hexi Corridor, and was a key passageway of the ancient Silk Road. It commands fascinating views of the boundless Gobi Desert and the snow-capped Qilian Mountains.

CN-1553140, sent by Françoise. 

Covering an area of 34 square kilometers (8 acres), the pass was built with rammed earth and is shaped like a trapezoid. The pass has a good layout with a three-storied gate tower, 4 corner towers and consisting of an inner city wall, outer city wall, trenches, and fortifications with walls around them. The walls are 10 meters (33 feet) tall on average and exceptionally thick.

 The initial construction began in 1372. It got its name for being located on the Jayu Highland between the Wenshu and Heishan mountains at the foot of the Qilian Mountains. Once abandoned, it was rebuilt, reinforced and garrisoned in 1539. In the Qing Dynasty (1636 - 1911) it became an office for checking merchants and travelers. - in: http://www.chinahighlights.com/greatwall/section/jiayuguan-pass.htm