The Dark Tower/WMG

The "Green Man" from Insomnia is actually Randall Flagg.
It's revealed in The Dark Tower VII: The Dark Tower that Flagg has no interest in the Crimson King's plan to pull down the Tower -- he wants to ascend the Tower and control it himself. For this, Flagg needs Roland to live long enough to foil the Crimson King, hence he needs Patrick Danville to save Roland. Since Derry was the Court of the Crimson King, and Flagg was ostensibly in the King's service, Flagg had to limit his insubordination to offering a crucial bit of advice to Louise. Even so, Flagg had to disguise himself to prevent the King from finding out.
 * That does explain why Lois was unable to tell whether the Green Man was good or evil: he was genuinely trying to help Ralph and Lois by giving them the earrings, but his reasons for doing so were rooted in his desire for multiversal domination. Hence his confusing moral aura.
 * Jossed by the last book. Insomnia is not a part of the tower universe; it's actually a deception in the same mode that Odd Lane is a glamour and a "wrong direction".

Both the original and the revised version of The Gunslinger actually happened.
They're just different cycles. "Ka like a wheel." Additionally, "continuity errors" between the other books are actually an indication that none of the Dark Tower books occur in the same cycle.
 * This means that the comic also happened, in a third loop.
 * The only problem with this one is that Allworld and Keystone Earth are the only constant universes. So despite the cyclical nature of Ka (and Roland's constant quest) it seems fairly implausable for it to be otherwise. This troper also thought that Roland's quest only cycled from the point just before he met Brown, meaning that there would be no good reason for him to use 70's slang terms or read magazines.
 * Jossed in the Coda of the last book. Roland is always sent back to the point to the Mohaine Desert (The man in black fled across the desert and the gunslinger followed), with no memory of what happened before. He is not sent further back in time where he could potentially change the mistakes of the past.
 * There is precedent for changes in events prior to Roland's trip across the Mohaine: In the Coda Roland gets sent back to the desert, but this time he has Cuthbert's horn. Roland did not have the horn in the previous loop; it had been lost at a point long before Roland entered the Mohaine.
 * That was an intentional concession on the part of the Tower. Roland almost seems surprised to find he has the horn but his past is practically the same.

All-World is Purgatory. Or Hell.
Jake and Father Callahan end up there by dying in our world. And there's also the whole cycle thing, which is echoed in the short story "That Feeling, You Can Only Say What It Is In French" (from Everything's Eventual), in which the main character is canonically Dead All Along (King's commentary on the short story confirms this).
 * Not sure about the re-write, but in the original version of The Gunslinger, Roland asks Brown if he believes in an afterlife. Brown states that he thinks "this is it." This has two meanings, one is that there is no afterlife, and this is the only world they have, or that they are living in the afterlife

All-World is Narnia as the magic runs out
At the end of The Last Battle, the children enter a new magical world they explore and have adventures in; once they find the center of that world they find a new world, as large and magical as the one that contains it for them to explore. At the end of the Dark Tower, inside the titular Dark Tower which is frequently referred to as the center of all things, Roland gets sent somewhere else, with the same quest: follow the man in black, find the Dark Tower.

Susannah didn't come from the same world as Eddie or Jake.
In the final book, it is explicitly revealed that Susannah disappeared in the real world; Eddie, on the other hand, comes from a world in which Co-op City is in the Bronx Brooklyn, rather than the Bronx, where it really is. Also, Eddie came from 1983 1987 and Jake came from 1974, while Susannah came from 1968; according to the last book, once you travel to a given time in the real world, you can never go back to an earlier time.
 * Whilst it's definitely possible that Susannah didn't come from the same world as Eddie or Jake, it's not totally certain that she came from our world. The A train doesn't stop at the Christopher Street station in the real world for one thing. This even gets mentioned in the sixth book.
 * Considering how many times the cycle may have happened before, it's possible that the real-world version of Susannah was drawn, had her adventures, and then left for a parallel Earth many iterations ago. That left an opening in the real world for the Susannah in the novels to claim her father's wealth there, even if she's not actually the same Susannah that inherited it there.

Robert Browning's Childe Roland to the Dark Tower Came is about the last cycle.
At least that's how I read the poem being printed at the end of book VII. Finally Roland makes it to the Tower and blows the horn. Finis.
 * Or, for the Downer Ending variant, Browning's poem was written about a previous iteration of Roland's quest, in which he still had the horn because he'd never left it behind. Browning wrote the poem at Roland's urging, because he was a previous iteration's Stephen King, who immortalized the gunslinger's story after Roland saved him from being run over by a speeding carriage. Roland then completed his quest and flashed back to the desert, only that time, the Tower took the horn of Arthur Eld away from him as a sign that the next round might end differently...

Carrie White from Carrie is a diclonius.
She has telepathic powers similar to the diclonius' vectors. She, like Lucy, was also bullied in school, and she goes on a killing rampage after getting upset.
 * Then where are her horns? In a detail-laden story like King's, you'd expect those to come up at least once.

Las Vegas, Heroes, The X-Files, The Stand and Knight Rider all take place in the same or parallel universes.
Both KR and Heroes have a Montecito, the setting of Las Vegas. Linderman was apparently running the casino, which would fit with the Montecito's high owner turnover ratio. It's possible the Company was killing or persuading the previous owners, using people with powers, in order to get their hands on it. Also, the Shanti virus, in an alternate universe, is what kills everyone in Stephen King's The Stand, except much earlier (This is supported by King himself, who said that The Stand universe was just an alternate version of ours). Possibly due to time-travel, or the Company experimenting with an early form of the virus. The Company coverups would also explain all the weird stuff that keeps happening to Mulder and Scully, especially if you've read the "Last One Standing" fanfic.

The Buick 8 and the Arrowhead project weren't the only portals.
The Gunslinger series mentions various portals between the two worlds. The special thing is only the Buick was planned to be a portal by the Darkman. Arrowhead was taken advantage of, while all other portals could be extremely difficult to open.

The Gunslinger is a video game protagonist, and the cycles of the Dark Tower represent his attempts at Save Scumming.
Or, alternately, he beat the game the first time around and either went into the bonus "Master Quest" or started a New Game+.
 * If this is true, it's definitely a Sierra game. "I made the game Unwinnable when I dropped the damn trumpet? And now I have to restore my saved game from the desert, seven disks ago? Screw you, Sierra!"

When the television adaptation is made, Roland will have the Horn of Eld from the beginning.
It'd give the writers a completely believable license to change things around: this is the next and possibly final cycle.

The Beast spoken about in The Dark Tower is actually the creature from It.
The Man in Black said that to speak of the Beast is to damn one's soul; it sounds like what happens when one looks into the deadlights. Also, what else but a Cosmic Horror could cause Randall Flagg to show fear?
 * Jossed in the final book.
 * Not so Jossed, especially considering that this is WMG.
 * Really?
 * In the revised version of book one, The Beast is referred to instead as The Crimson King.
 * It's more likely that the Beast is Dis, the ultimate embodiment of the Red/Random/evil, in the same way that the Tower is Gan, the ultimate embodiment of the White/Purpose/good. The Beast seems to be part of the Tower, or trapped within it in some way; we know that Dis is imprisoned in the top room, and the Crimson King is merely an aspect of it.
 * In It, the driving force behind it was the deadlights (the red) circling outside the gates of the universe while the other that help the kids after the turtle was the white. Since Pennywise was the physical embodiment of the deadlights, It was the most powerful creature in King's universe. All others that followed were decedents/creations of Pennywise or at least a varintant (like the Crimson King's Spider-god mother).
 * This troper recalls reading a theory that Dandelo (and perhaps even the Crimson King) were hatched from It's eggs.

There are several minor characters with the ability to become Gunslingers in the books.
As well as all the named characters who become gunslingers, there are others who could potentially have become gunslingers. King describes the "cold, emotionless" feeling that drops over people when they are about to kill with a gun, and several characters exhibit this trait. Examples includes the cop who exploded Jack Mort's lighter in New York, Susan Delgado, and the Sisters of Oriza. There may be others...

The revised edition of the first book starts at the end of the last book.
The continuity errors revised in the books were also done in universe, when Ka corrected itself after the mistakes it made in the last cycle.

The cycle will end - when Roland can renounce the Tower at the proper time.
Per the final book of the series, after Algul Siento is destroyed, the remaining Beams are saved and will even restore those that were broken. In addition, it's revealed that the Crimson King is trapped on his balcony of the Tower and completely unable to advance, leave, or affect it in any way. Effectively, the destruction of Algul Siento marks the salvation of all universes. However, Roland feels incapable of renouncing his quest - he must reach the Tower, not for anyone's sake but his own.

If Roland continues his quest for the Tower after this point, it marks a failure on his part - he threatens the fate of all things for the sake of his own personal quest, and opens the possibility for the Crimson King to acquire the keys to the Tower. It's even possible that his entry of the Tower weakens it again somehow, which is why he is sent back to the desert, as the Tower attempts to rewrite the damage done. The only way for all worlds to be safe will be for Roland to complete the truly noble aspect of his quest, the preservation of the Tower - perhaps even the destruction of the Crimson King - and having stood at the steps of the Tower, have the strength to turn around and leave the Tower be.

Good luck on that, Roland.
 * There's a suggestion, though, that this is how things eventually turn out; in the very last lines of the book, as Roland sets out again across the desert to find the Man in Black, it's suggested that the timeline has been altered -- Roland has the horn again, which he didn't have in the previous cycle. So it does indicate Roland's iterative attempts to find the Tower are moving him towards renouncing the Tower.
 * Supported given that in the book version of Roland's quest, it is the roses themselves that "blow his horn" and open the tower for him. It's an image tied up with him being compelled to enter the Tower. Possibly when he has the horn and blows it, there is no compulsion from the Tower for him to enter.

The following are included among the Breakers (or, if dead, would have been)

 * Patrick Danville (Reality Warper)
 * If Pat were a Breaker he'd have had to escape somehow: he spent years before their operation was shut down locked up in Dandelo's cellar.
 * John Coffey (Healing Hands)
 * Charlie McGee (Pyrokinesis)
 * Carrie White (Telekinesis): although note that Dinky Earnshaw actually refers to "being like Carrie at the Prom", which suggests she was, at least in his home universe, a fictional character only.
 * Danny Torrance (Mind-Reading)
 * Dick Hallorann (Clairvoyance)
 * Johnny Smith (Clairvoyance, psychometry)

The cycles of Roland's journey are entirely different each time.

 * It just feels to me that if time was reset entirely, that would render the "Susannah in New York" epilogue useless, and the warning in "Found" unnecessary because Roland is now able to start over and perhaps prevent everyone's death by accident. Instead, it makes more sense that only Roland's age and the life of Randall Flagg (because he's Randall Flagg and he's too demonic to really die) get reset. Part of Roland's death-curse, put upon him either by the Tower as punishment or by the Pink Bend o' the Rainbow when it glamoured him into finding the Tower, is that every time he goes to the Tower, he finds a new ka-tet, and they are all killed along the journey. He has probably already killed fifty or so people in his ka-tets by taking them on previous cycles. Susannah is the first one to ever escape, hence she lives happily ever after. That is Roland's true curse.
 * It's not a full reset every time he enters the Tower. The last book makes it clear that it's only for Roland that time is reset. He's forced to relive the experience of his last ka-tet endlessly. The only difference is that during this particular experience, for whatever reason, the Tower saw fit to give him back the horn he lost at Jericho Hill, and therefore there's the chance that at the end of the journey this time round he'll blow the horn rather than having the roses blow it for him, and the curse will end as per Robert Browning's poem.

The Man In Black in the desert is not Walther, but the embodiment of the Tower itself.

 * The Tower is there in the form of his nemesis to warn Roland to turn away from his obsession of reaching the Tower, while Jake is in a sense the anti-thesis of the Tower (Not of Gan, but of what Roland makes out the Tower to be). When Roland will finally choose Jake over the Tower, the cycle will end and Roland will find his peace the same way Susannah did. But every time he chooses the Tower, he has to relive all of that pain and grief he has endured for hundreds of years, while fighting the ghosts of his long dead enemies.
 * Jossed in the last book. Randall Flagg identifies himself as the Man in Black and explains how he basically cast a spell on Roland to escape and deceive him into believing he was long dead.

The child Breakers in Black House were working on a different Beam to that of Shardik and Maturin.

 * Nobody in Algul Siento ever mentions the Big Engine which the "party" in Black House destroyed. Therefore it was an effort at breaking one of the other Beams, possibly even the one of Elephant and Fish which collapses in Song of Susannah depending on how heavily it was damaged before the Coppicemen intervened.
 * It is mentioned in 'Dark Tower' that the Crimson King's forge (seen in the vision in 'Song of Susannah') has been shut down. This was the big engine, the machine CK used to spread his malice and corruption through every level of creation.

There is another, bigger cycle.

 * The cycle we see in the books, that starts at the beginning of The Gunslinger and loops back at the end of The Dark Tower, is a relatively small one. Perhaps it's just a tiny part of an even bigger cycle, going from the beginning of the multiverse to its end? Once Roland has completed his final Dark Tower cycle, the multiverse should continue on as usual (albeit with evil banished, or something). But when the universes of Mid-World and Keystone Earth eventually die (in a Big Crunch or something), the entire multiverse will be rebooted. This explains the difference between the Dark Tower books, comics and upcoming films: they're part of different cycles all right, but the events flashed back to in Wizard and Glass take place outside the Dark Tower cycle, and must therefore be part of a different, larger cycle if the differences in versions are to be accounted for. This also explains how the book, television and comic-book versions of The Stand can all exist, despite the fact that they all have different versions of Randall Flagg in them: it's impossible that there are multiple Flaggs running around the multiverse (there's no way this guy could have twinners), so the different Flaggs must be part of different multiversal cycles.

Patrick Danville is the son of Roland Deschain and Susan Delgado.

 * Dying is one way for a person to travel from one world to another (eg Jake). When Susan Delgado was being burnt in Hambry, perhaps the embryo she was carrying was transferred to the womb of another woman in another world: Sonia Danville. The supernatural forces surrounding the ancient evil of the charyou tree may have caused the dimensional rift. Sonia bore the child – the biological son of Roland and Susan – thinking that it was hers. When Ralph Roberts is reading Sonia's mind in Insomnia, he finds out 'Patrick, that's his name. She calls him Pat. He's named after his grandfather.' Patrick Delgado was the father of Susan Delgado, meaning that any child of Roland and Susan would have been the grandchild of Patrick – who is often referred to as Pat. It seems that Sonia is aware of her child's origins on some subconscious level. And of course, Patrick Danville's Mid-World heritage would go a long way towards explaining his supernatural abilities and cosmic significance. Besides, it's too damn depressing to think that Susan and her baby just died.
 * Alternately, Sonia Danville is Susan Delgado's Twinner (see Black House for more info, though if you're here I imagine you already are familiar at least in passing), and Susan's unborn child would have been Patrick's (Twinner, that is, not child).

Arthur Eld is the son of Randall Flagg and Nadine Cross.

 * This theory operates via the same mechanism as the one above. When Nadine Cross manages to goad Randall Flagg into throwing her out a window in The Stand, she and the child she was carrying apparently die. But if it's possible for unborn children to be transferred from one world to another upon death in the same way Jake was transferred, perhaps the offspring of Nadine and Flagg ended up in the body of a Mid-World woman – and since time is irrelevant when travelling between universes, it's entirely possible that the child ended up in the body of a woman in the distant past. This woman, whose surname was Eld, gave birth to the child and named him Arthur. The boy was enormously talented, and rose through the ranks to ultimately become the King of All-World. He led his people to victory against the wizard Maerlyn – who is, in fact, his grandfather, as the comics reveal that Flagg was Maerlyn's bastard offspring.Arthur has two children during his life: one was a son by a mortal woman whose surname was Deschain – this child was the ancestor of Roland through twenty-nine generations. The other child was by the demonic Crimson Queen, whom Arthur was tricked into sleeping with by Maerlyn's manipulation. Therefore: Maerlyn fathered Randall Flagg, who fathered Arthur Eld, who fathered the Crimson King and also sired the line which begat Roland Deschain. The Crimson King and Roland Deschain then co-fathered Mordred Deschain, in whom was reunited the two lines of Eld. Mordred capped the whole thing off by eating his ancestor. And if we combine this theory with the above one, Patrick Danville is also a descendant of Maerlyn, Flagg and Arthur, as well as being the half-brother of Mordred.

The books depict the 19th iteration of Roland's quest.
The number is a glorified loop counter. The problems it causes in Tull (revised edition) are because it reveals to the townsfolk just what's going on--Eternal Recurrence is a tough pill to swallow. Instances of 19 in other books can be explained by All-World being one of the Keystone worlds, with the counter diffusing through the multiverse during each loop.

Roland has the Horn in the new iteration because he saved Stephen King in Keystone Earth.
Everything's set in stone in Keystone Earth, isn't it? With Roland presumably going back in time to the moment in the Mohaine Desert, the changes he made to Keystone Earth persist, and as such the Tet Corporation already exists at this point. Stephen King is also alive, and, knowing on some level what happened to him, he rewrites the story so that Roland already has the Horn.

Roland's quest will end...
... when Roland chooses Jake over the Tower and saves him, instead of following the Man in Black. If he doesn't let him fall to his death, the rest of his journey goes an entirely different way, and the cycle is broken.
 * At that point in the cycle the breakers would still be active, so the tower would fall. It would break the cycle but not in the right way.

Kindness will end the cycles.
There are several different people who could've changed things if they'd been allowed to live. Some changes--Mordred being accepted when he was born, say--are large and obvious. Others are less so, like that one can-toi in the Algul who seemed to be Becoming in truth. Even Lamla o' Galee, who begged for clemency, might have been a tipping point. Saving enough of the lost ones might well win the day.
 * Morded ponders what it would be like to be accepted, but comes to the conclusion that even if he was he would never yield to having Roland as his Dihn/king, so even if they did accept him he would want to be in charge of everyone.

Everything that has happened to Dark Tower's incarnation of Stephen King actually happened.
The money he paid the man in the accident wasn't so he could buy and destroy his car. But also keep him quiet about his dealings between himself and Roland.
 * creepy...

Roland IS The Dark Tower
The Dark Tower has three incarnations instead of two, the Tower itself, the Rose and Roland himself! Roland is the human incarnation of the Tower, so what he's really searching for is himself (deep huh?)