Stephen King is largely known for his horror novels, many of which have been made into blockbuster movies. Stories like The Shining, Carrie, It, The Stand, and Misery have contributed a lot to pop culture over the past four decades, but the author's largest and most important work hasn't been adapted to film until now. The Dark Tower is the long-awaited movie version of King's sprawling 8 novel fantasy epic, and fans are hoping to see the Gunslinger reach the eponymous Dark Tower at the film's conclusion. But what's in the Dark Tower, and will fans ever get to see it?
In the complex mythology of the series, the Dark Tower is the nexus of space and time. It's a literal building, a 600-foot tower made of black stone, that balances all of the realities in existence. The tower is held in place by six giant invisible beams that stretch out into the dimensions they connect. The beams themselves are living entities, as old as the universe, with every one terminating in a portal on each side, with an animal guardian at each portal (you getting all of this?). If the tower were to ever fall, then it would mean the end of all universes. It's the destiny of Roland, the story's protagonist, to prevent this from happening by reaching the tower before the evil Crimson King can bring about its destruction.
The tower's physical representation can only be reached from one reality — All-World — and there is just one door which can be used to access the tower. But as far as what's actually inside the tower is concerned, well, that's something that's never really been made clear. Roland is under the impression that some kind of god or demon lives at the top of the tower, and he feels compelled to question it. But since he never actually makes it up there in the books, it's unclear if he's right in this assumption.
One popular theory that's alluded to in the books suggests that the tower itself is a physical manifestation of Gan, essentially God in the Dark Tower mythos. A separate fan theory on Reddit claims that the top of the tower contains a time portal, which contributes to the endless loop that Roland finds himself in. But really, no one knows for sure what's up there, as it's never been depicted in the story.
What's inside the Dark Tower isn't really important. The main thing to know is what the tower represents — the basis of reality — and that keeping it intact is extremely important if you value, you know, all of creation.