The lone door on this level is a TrapDoor that turns into a Yellow D. Note that once you grab the Dark Crystal, you can't leave the dungeon without losing Kain, or fighting the EvilWall. If you want to do any Underworld sidequests with him, this is your last chance to turn back.
The EvilWall is incredibly nasty. It's basically a timed battle, and time is short. You can win pretty easily with Haste and Bersk (using Hermes and Bacchus Items if you have them to spare) and having Rydia cast Bio. The EvilWall will inflict a lot of Gradual Petrification, so Rosa should be ready with Esuna. If it starts killing people, you're almost out of time.
You can't use Exit in the Sealed Cave, but you can Warp on most floors, so you don't have to walk out if you don't want to.
The EvilWall is incredibly nasty. It's basically a timed battle, and time is short. You can win pretty easily with Haste and Bersk (using Hermes and Bacchus Items if you have them to spare) and having Rydia cast Bio. The EvilWall will inflict a lot of Gradual Petrification, so Rosa should be ready with Esuna. If it starts killing people, you're almost out of time.
You can't use Exit in the Sealed Cave, but you can Warp on most floors, so you don't have to walk out if you don't want to.