The world has drowned. It seems to have drowned long ago.
There only thing left is a castle. It is a broken thing, worn smooth by the tender ministrations of the ocean. It sits atop a jumbled pile, composed of more castle. Perhaps it is castle all the way down?
It is a dungeon of course, and every level is larger than the level before it.
There are three towers. One straight, one crooked, and one fallen.
The princess lives in the crooked tower. On some fundamental level, she knows that this castle is for her. It was built to protect her, or preserve her. They worked hard to insulate her from the apocalypse.
She has vague memories of a court, a king, a wizard, a tree, a chain, and eight women in a circle, grunting as they gave birth in perfect synchronicity.
She does not know what these things mean. She knows that a great many people died so that she may live. This used to make her very sad, and she used to cry a great deal. (The barnacles heard her many laments.) But now she is not so certain. The ocean is certainly large and beautiful, and all those dead people: were they good or wicked? Nothing is certain.
She does not need food. For a while, she ate fish because it seemed like something that she should do, but defecation became more and more odious, and so she stopped eating altogether. She no longer gets hungry, although sometimes she worries that she might be.
She has no heartbeart, either. There is something small and musical in her chest. Once she tried to dig it out, but it hurt so much (and there was so much blood) that she stopped.
Sometimes she dreams about her old days in the castle. The time the cook gave her shortbread cookies by rolling them across the floor. The maid with the blue blotch on her chin. (She was ashamed of it, but the princess thought it was beautiful.) The stableboy who once drowned a sack of kittens in front of her, and was shocked when she was horrified by the act.
And after those dreams, she finds those people. They come out of the ocean.
These are the player characters.
Are they dreams? They might be. She has certainly seen them die, and then they returned later, after she dreamed of them.
They come from the ocean naked, full of half-memories and an undying love for the princess. They love her and would do anything for her.
She sends them into the castle below, to see what can be saved.
This is all the result of an attempt to survive an apocalypse. Cities and castles were all scooped up and heaped into a mountain, then something like a last-minute arcology was attempted. Success was. . . very incomplete. Sabotage, or something like it.
This can be run as a stand-along campaign, an actual world where only a single castle survived.
Or you can run it as a megadungeon location out in the middle of the ocean, with a princess who merely thinks that the world ended, and that she dreamed you and your ship out of nothing.
Let's keep it open-ended, shall we?
In the beginning, just finding food and equipment will be challenging. Expect the first couple of forays into the castle to involve scrabbling to obtain a knife. A sword is a real treasure. It becomes easier to re-equip in the future. You can store surplus gear in a chest, near the princess.
Does that mean that money is worthless? Nah, I'm sure there are people who will still sell you things, down the dungeon.
There are NPCs in the castle, and you must find them and recruit them. Getting them to fish is enormously useful, since they can restock your food supplies when you visit them.
You can also explore the other two towers. The best way to do this is to get over there and then run a rope bridge over there. (You don't want to get in the water. There are shark-men-things in there. The princess hears them talking on moonless nights.) The other two towers lead to shortcuts down to deeper levels.
You'll also want to make friends with the dungeon. The dungeon likes to be clean, to be repaired, and for its rooms to be used for their original purposes. The dungeon hates to be broken or ignored. Things the dungeon can do if it likes you: unlock a locked door, create lights, shunt you up/down a level, give you previews and maps.
The crowning castle will not be hard to find. It's near the top of the heap, where all of this madness was first enacted. You can find the sorcerer king who orchestrated all of this. He knows that he is a memory, an endlessly looping tape. He will always ask about his daughter, then fall silent. His halls are magnificent: vaults of filigreed stone and laughing fountains. His court is in disarray. They want to know when his spell will take effect. When will they be saved? There are a great many assassins in the adjacent room. They are waiting for the signal from their leader before they attack. The sorcerer-king brought this upon them. The sorcerer-king is a liar. The sorcerer-king must be killed.
The surrounding city. A great many people. Too many people! They are fleeing the coming waves. They are elbowing you, shoving you aside. It is difficult to go anywhere. There are gallows and there are shops and there are lovely painted ladies. Everything is wrong. There are tiny hats inside the pies. Every shoe that the cobbler offers is a well-disguised prostitute in a clever disguise, hiding from the town watch. The guards are empty armors, shaking with fear. They wanted to run, but where would they run to? People engage in the business of apocalypse: fleeing, fucking, laughing, weeping, desperately holding loved ones.
The surrounding countryside. All wrapped up inside itself. Fields of corn that grow on the floor, walls, and ceiling. The sounds of cows, but no cows. Shepherd-things who have become one with their flock. Eggs upon eggs--it is unwise to break a single one. Priests of the old countryside, blind eyes witnessing vistas that you cannot, speaking of the coming apocalypse. The burnt granary, tessellated endlessly, always with the same dying child in the middle of every room.
There is a place that was once a plague ship. They are locked behind a gate, grasping and gasping, but if you would explore that wing of the dungeon, you must open that gate.
There is a place that was once a battleground. They are fighting off an enemy army, come to kill the sorcerer-king before he can kill the world. The walls are spears. The ceiling is banners. A plain has been distilled into hallways.
Everyone is dreams, but some are more permanent and adaptable than others. These are the ones you must recruit.
He is the opposite of the Princess. He wants to end it all. Look at these unhappy dreams, these trapped ghosts. Is this a world worth preserving? Wipe the slate clean, let it go. Only when it is all plowed under can something new be planted.
His dreams are dark and thick and very, very sharp.
You will meet him, and when he talks he will seem very reasonable and very wise. Perhaps he is.
Perhaps the castle is not the only thing left. Perhaps there are still ships out there, endlessly circling the globe.
The ships are looking for the castle. Some of them will find it.
The ships will be full of madmen, or saints, or seers. They will come to the castle seeking salvation, and they will not find it. They may pillage or they may trade. They may even join you, if you have the comforts they seek.