Pinyin: y and f dāng guān, w à n f m ò k ā i.

Explanation:

It means that the mountain is high and dangerous. If one person guards the pass, 10,000 people can't get in. Describe the terrain as very dangerous.

Source:

"A Brief Training of Huainan Zibing": "One person guards the pass, and a thousand people dare to cross it." Jin Zuosi's "Shu Du Fu": "One person guards the customs, and ten thousand people are unwilling."

The most obvious example: Jianmenguan, Sichuan, has a dangerous terrain and is easy to defend but difficult to attack.