Dead Cells

To get to the cavern, you need the cavern key. It's actually at the very start of the game.
Do you know that giant hole you can enter and when you go in there is a big lava pit? That is where the key is. You see a door over the pit that is locked. You need to be able to detach from your body. Shoot your green blob thing as far as you can. I had to jump and then do it to make it. You want to go PAST the door, so you are under then hit the A button to jump. Time it right and make it to the wall under the door/lock. Then move up and unlock the door. Go back to your body and jump across.

Then you need to make your way to the Graveyard where you will find the door to the Caverns. (NOTE: you also need all the runes)

Recommended Path:
Prisoners’ Quarters --> Toxic Sewers --> Anicent Sewers --> Insufferable Crypt -->Graveyard -->Caverns

You can go this way too, but I think you need 3 boss souls to get from Ramparts to the Crypt.
Prisoners’ Quarters --> Promenade of the Condemned --> Ramparts --> Insufferable Crypt -->Graveyard -->Caverns

Here is a copy of the most recent Dead Cells map
https://preview.redd.it/yj3o9m6ftjf91.png?width=2926&for...