Jiayuguan Great Wall, originally constructed as the last major stronghold of Ming Empire, once faced boiling Gobi desert and vast terrains towards Central Asia.  It's strategically located at the narrowest point of the Hexi Corridor on the Silk Road and now becomes the greatest wonder of the world.

This episode is also available as a blog post: https://bauhiniahk2014.wordpress.com/2017/09/18/jiayuguan-pass-%e5%98%89%e5%b3%aa%e9%97%9c-first-and-the-greatest-pass-under-heaven/