If you like your fantasy worlds full of ominous black structures and Watch Bikini Pirates (2006)universe-vomiting turtles, you're probably going to enjoy The Dark Tower.

Chances are you've already seen the trailer. For anyone not familiar with Stephen King's mighty eight-book series, though, you may be a bit fuzzy on some of the details. Where does the story take place, for instance? And what's the significance of the tower?

From Mid-World to the sprawling concept of the Multiverse, we've broken down a few of the key points.

First, what actually is the Dark Tower?

In short, it's a structure that acts as the centre of all time and space. It quite literally holds the multiverse -- which we'll get onto later -- together. It's also the mystery at the heart of TheDark Towerseries; we're told that it's in danger of collapsing, and that main character Roland Deschain (Idris Elba in the films) may be the last thing that stands in the way of this; but we don't know exactly what's causing the collapse, or how the Gunslinger plans to stop it.

Where is The Dark Tower going to be set?

The books largely take place in two different settings: a place called "Mid-World" (this is where Roland lives) and a version of our own world known as "Keystone Earth." The action swaps between the two, which are linked to each other by doorways that act as portals. From the looks of the trailer, the movie will follow suit.

What is Mid-World?

The home of both Roland and the Dark Tower itself (in fact it's the only world in the multiverse in which the Tower is actually present in its proper, Tower-y form). Mid-World (also known as "All-World") is sort of like a post-apocalyptic version of our world in which things have regressed to a kind of crumbling, Western outlaw state. There are monsters, weird civilisations, and vast planes of nothing much at all. It's divided up into three areas: In-World is where Roland is originally from, Mid-World is where the bulk of his adventure in the books takes place, and End-World is his final destination -- the place where the Tower is located at the very centre of everything.

So what the sweet hell is the 'multiverse'?

OK, this is where everything gets a bit complicated. The thing is, many of Stephen King's novels -- and particularly the characters that feature in those novels -- are connected. In much the same way that the Dark Tower holds the universe together, King's Dark Towernovels hold his entire fictional world together. Remember the mention of Pennywise (the clown from IT) hidden in the first Dark Towertrailer? Well, that's a sign that the movie will be following this pattern; the universe in which ITtakes place is the same universe in which the Dark Towertakes place.

Everything in the King canon is linked in some way, and the multiverse is what they're contained within.

But how does the multiverse actually work?

Right, now things are going to get really confusing.

So the Dark Tower is the centre of everything in the multiverse, and it's supported by six "beams," each of which are watched over by two animal guardians (one at each end). If you've ever read ITyou'll know that there's a giant, mystical turtle in the story that was supposed to have vomited out our universe; this turtle (called Maturin) is also one of the beam guardians of the Dark Tower. Everything's connected, see?

The various worlds contained within the multiverse also overlap and merge with each other in some cases. Although the Tower itself isn't present in Keystone Earth, for instance, it does have a physical representation: a rose found in New York City in a vacant lot.

Oh, and just for some added meta-fun, Stephen King himself even appears as a character later on in the novels (that's a long way down the road, though, so it's unlikely he'll make a cameo in the first film).

TL;DR: the multiverse is a fictional, multi-dimensional universe created by Stephen King. It contains various worlds, and various versions of these worlds. Worlds overlap and merge, the Dark Tower is at the centre of everything, and it's all pretty confusing. For the purposes of the film, though, there's only really two worlds to worry about: ours and the post-apocalyptic one that holds the Tower.