Image is from HERE
This is basically a framework for a Dungeons & Dragons or Lamentations of the Flame Princess campaign, or the fantasy RPG of your choice. The idea is that there's a magic Tower which (possibly among other things) allows travel between different "worlds" or planets or universes or alternate realities or...something. No one seems to be quite sure, especially since the Tower is imperceptible and intangible to most people - the few who can see and interact with it have little in common, and may be "chosen" at random - and there are conflicting reports about exactly where the Tower is located. Pretty much everyone who even knows and cares about the Tower in the players' starting world is working from old legends and theories and such. The few who have entered the Tower and returned report that the way up is blocked, and the way down leads to some strange, hostile places.
Oh, and the legends claim that God lives at the top of the Tower, and He loves to grant wishes for anyone who can make it to the top to come see Him.
The players begin in the world of Floor 1. They quickly discover that they are among those "chosen" to be able to perceive the Tower, which they find in the middle of an unsuspecting town. If the players choose to enter the Tower, they find that the staircase to Floor 2 is sealed off by a magical barrier, but the stairs down to the first basement (Floor B1) are unobstructed. The first floor also contains a clue as to how the players can find the key to unsealing Floor 2, located somewhere in their own world on Floor 1. If the players ever make their way to Floors 5, 10, 16, and possibly 23, they will similarly find the way further upward sealed off, with keys to breaking the seals in those worlds.
The players can set their own goals if they want, naturally, such as exploring one or more worlds or getting involved in various conflicts throughout the multiverse or discovering the secret meaning or purpose(s) behind the Tower or just killing things and taking their stuff. But the most obvious possible goal would be to climb the whole Tower and meet God, and maybe get some wishes granted. If the players are feeling especially fond of JRPG tropes, or just feeling really Metal, they could even try to kill God and take His place or something.
One more thing: the magic "seals" between Floors 1 and 2, between Floors 5 and 6, etc. actually only block traffic in one direction, namely upward. Strange people (and stranger creatures) sometimes travel down the Tower and find themselves unable to return from whence they came. The starting world on Floor 1 is home to some such travelers, as are most or all of the other worlds. A really tricky DM could add seals on Floors B3, B5, and B6, so that players who explore downward cannot return upward without completing some kind of quest(s) to break one or more seals. I listed these floors as "once sealed" below, but there's no reason you couldn't have those seals still be active.
You could use all kinds of campaign settings and modules to flesh out the Tower's multiverse. If I were to use the rules and materials from LotFP, for example, here's how I would probably arrange the various worlds of the Tower:
Floor B6: This is the base of the tower, rooted in the Veins of the Earth. (Maybe the bottom of the Tower could be the Deep Carbon Observatory, hanging from the ceiling in a giant cavern in the Veins.) There could be other Towers accessible throughout the Veins. Other worlds than these. There is evidence that the Door to Floor B5 was once sealed.
Floor B5: Exits to Narcosa. Drug-using NPCs who originally hail from higher in the tower constantly make lame jokes about "The Underground Drug Trade." (Ha ha! Get it? Snort.) There is evidence that the Door to Floor B4 was once sealed.
Floor B4: Exits to The Gem Prison of Zardax. If there used to be another world here (outside of the Prison), it's gone now, replaced by the technicolor void.
Floor B3: Exits to Towers Two. There is evidence that the Door to Floor B2 was once sealed.
Floor B2: Exits to the flower plantation of The Pale Lady.
Note that there can sometimes be alternative methods of travelling between worlds other than climbing or descending the Tower, but that these "shortcuts" may be dangerous, unpredictable, or time-sensitive. For example, you could allow the players temporary access to this world as per The Pale Lady, but if they miss their window of opportunity to return the way they came, they may have to climb the Tower in order to get back home.
Floor 1: Earth, or Oerth, or Mystara, or Avalon Hill's Outdoor Survival map, or whatever you want the "normal" world to be. Door to Floor 2 is sealed.
Floor 3: Exits to a town known as Pembrooktonshire, where the residents insist everything is practically perfect just the way it is. Heavenly, even. After all, didn't you climb up to get here? And isn't this the top of the tower? (They all seem to be convinced it is, even though there's clearly an unblocked path to Floor 4.) Some "halflings" were recently run out of town - see Lamentations of the Gingerbread Princess. There are also dangerous "monsters" in the mountains - see No Dignity in Death and Hammers of the God. Could also tie in nicely with the adventure "In Heaven, Everything is Fine" from Forgive Us.
Alternatively: If you want to make Pembrooktonshire and its surroundings the starting world on Floor 1, you could have Floor 3 exit to Hot Springs Island.
Floor 4: Exits to Weird New World. The elves are not native here; they descended the Tower from another world, and now believe they are in Hell.
Floor 5: Exits to The Driftwood Verses. Door to Floor 6 is sealed.
Floor 7: Exits to the Isle of the Unknown.
Floor 10: Exits to A Red & Pleasant Land. Door to Floor 11 is sealed.
Floor 12: Exits to the city of Vornheim.
Floor 13: Exits to Bad Myrmidon. Currently undergoing a bad drought.
Floor 14: Exits to Scenic Dunnsmouth. If something is done about the Time Cube, it starts raining again in the world connected to Floor 13.
Floor 16: Exits to Carcosa. (Maybe the titular city is abandoned due to something involving the Tower? Maybe the Tower is in the city, or is the city, on this Floor?) Door to Floor 17 is sealed.
Floor 21: Exits to Qelong.
Floor 23: Exits to The Chaos Gods Come to Meatlandia. Maybe throw in some material from The Hateful Place, since things have gone so incredibly wrong in this world.
Things could go one of two ways from here. Either the door to Floor 24 is sealed, and the Tower continues upward for a while longer, in which case we have...
Floor 31: The top floor. This is supposedly where God dwells.
...or Floor 23 is actually the top floor...inside the Tower, that is. In this case, Meatlandia is actually the same world as the one where the campaign started out on Floor 1, except far in the future. Now a new path can be traveled up the outside of the Tower, leading to...
The Roof (a.k.a. Floor 24): This is supposedly where God dwells.
Either way, if the party actually makes it to the top of the Tower, it should be within the realm of possibility that they can kick the ass of whatever so-called God dwells up there and take His Most Holy Stuff. It doesn't have to be easy. Success could be so remote that calling it a long shot is like saying the trip to Alpha Centauri is a long flight. But if they make it this far, I think I, as the referee, owe it to the players to prevent the ultimate boss fight from being completely hopeless. I mean, look at the video game I swiped this Tower business from. Take it from Jackie Kashian: deicide is a great way to end a fantasy game.
If the party vanquishes God or "God" or whoever the man in the funny hat turns out to be, a big magical door is revealed. Where does it go? Beats me.
Image is from HERE
P.S. I would assume that all PCs are among the "chosen" who can find and enter the Tower. As for hirelings and other NPCs that the players may want to bring with them between worlds, you could handle this in a few ways. The DM could roll randomly to see if a given NPC is "chosen." The players could discover a way to grant "chosen" status to normal people (at some kind of cost, of course). The DM could decide that being "chosen" requires being controlled by a real-life player, i.e. being a PC, as a sort of "meta" element to the game, and any NPC who becomes a PC suddenly becomes "chosen." Or you could use a combination of these elements, or do something else.