Quick answer: You get the key by clearing a Gold Mask and 2 Iron Masks on a deck below Shrine Outer Gate. Press E at the red gate near that bell to open it.
The whole route stays inside the Mountain of Imprisonment. Most of it is drops, a wall slide, and quiet paths. There is one real fight at the end, so leave the bell with full health and full gourds.
How to reach the key deck
There are no fights between the wall slide and the yard. If you stay on this path, you reach the deck at full health.
Getting the Shrine Gate Key
Three fighters guard the deck: one Gold Mask accompanied by 2 Iron Masks. Enter with your health full and all gourds full.
Clearing the whole group gives you the key. You don’t need to pick anything up. The item card appears on screen by itself and reads:
“The key to the mountain gate on the only path to the shrine. It was hidden within the caverns by Huangxing’s personal guards.”
How to open the Shrine Gate
Use bell travel to return to Shrine Outer Gate. In the yard visible from the bell is a large red double gate fitted with lion-head rings.
Without the key, pressing E makes the gate rattle and nothing more. Once you have the key, press E to make your character push the doors open, revealing a fenced path with steps. It remains open even after death and a respawn at the bell.
Rest and refill at the bell before entering. An archer on the road past the gate deals heavy ranged damage. Walking past him can drain nearly an entire health bar in roughly 14 seconds.
The winch and cage above the deck
A ladder on the deck leads to an upper path; climb it when the Space prompt appears. An E pickup at the top grants Remnant Dust. Beside a hanging iron cage, a winch can be operated with E; your character pulls its lever to lower the cage. Nearby winches also display ring markers.
Don’t jump from the top of the ladder back down to the deck, because the fall kills you. To get back to the gate, use bell travel to Shrine Outer Gate.
Huangxing’s Betrayal note
The note is an Anonymous Note on the plank ledge you reach after the cave. Press E by the crate and jar to read it. Part of it reads:
“By Master Huangxing’s order: all Powderkegs are to be reassigned to the Prison Cave immediately! If another accidental blast causes trouble, the entire unit will share the punishment!”
It also says where the key went:
“The mountain gate key is also to be transferred to the captain of the cave guard!”
Closing the note gives you Remnant Dust + 5,570.
