I'm stuck in a bit of a rush trying to prep a One-Shot set in Ravenloft. The only hard parameters are:

1. Can be finished in four hours of gameplay
2. Appropriate for 3rd level PCs

The one shot is in a series of one-shots centered around a party of people who didn't know each other and woke up in Ravenloft with a device that sends them into a domain of dread unit they can figure out how to power it up and send them elsewhere (like sliders). In a previous one shot, they powered it up with the gross ichor they got from defeating a very important horrible creature.

The rough idea that I've come up with so far is that the PCs are stuck on a micro-island of terror that contains a keep that's sinking into the sea of sorrows. To keep PCs moving, I'd like the tower to continually sink so that PCs are on the clock to get up through the tower to the top. I'm also toying with the idea that occasionally the tower actually does collapse, killing the party and causing some kind of Groundhog Day scenario (perhaps with a weird twist that their dead bodies remain or become undead and are added to the castle).

Anyway, I need to flesh out two things:

1. "What's the deal" with this Castle
2. Some specific kinds of things inside

One sort of idea that I have is that the Keep is stuck in a cycle of collapse as a way to torture its original lord, who, in a previous life on the Prime Material plane, ordered his guards to lock everyone in the keep rather than let them evacuate during a cataclysmic flood. So, now he lives and re-lives this catastrophe over and over while adding travelers to his ranks whenever he can.

In terms of specific rooms, I'm thinking there's some bottom level sewer-type area, then perhaps some kind of dungeon, maybe a great hall where the guests are prisoners? I feel like each situation needs some kind of horror-themed twist, challenge, or hazard.

For the final encounter, the party presumably has to face either the lord of the keep or some sort of "champion" who guards him.

Any advice or inspiration would be great. I don't need much to fill the four hours, it just has to be buttoned up in a way that makes it fun and weird.