The metaphor is an eight-character idiom with one enemy ten thousand.

One man keeps the pass, but ten thousand people can't cross it-(referring to the pass) it is a strategic place and difficult to enter.

You're right.

"Definition" refers to the mountain is high and dangerous. A person guarding the gate, ten thousand people can't get in. Describe the terrain as very dangerous.

[Language] Tang Li Bai's poem "Difficult Road to Shu" reads: "Although the Diaolouguan is strong, it is difficult for ten thousand people to break it."

When one person guards it, all people are invincible.

[Usage] Complex sentence patterns; As an object, attribute and clause; Describe the terrain as very dangerous.

Although the Dagger Tower Pass is firm and stern. (Tang Li Bai's poem "Difficult Road to Shu")