Not words, not pictures can explain what Mustang is.
If you want to know more Here you can find a good (video) introduction.
From the Huffington Post:
“Mustang, a former kingdom in north-central Nepal, is home to one of the world’s great archaeological mysteries. In this dusty, wind-savaged place, hidden within the Himalaya and deeply cleaved by the Kali Gandaki River—in spots, the gorge dwarfs Arizona’s Grand Canyon—there are an extraordinary number of human-built caves.”