Stephen King’s world-traversing fantasy epic is a fan favorite, but even if you’ve read all eight volumes of the Dark Tower saga, you can probably pick up a few new nuggets and theories about the sweeping work.

1. Robert Browning’s Poetry Inspired the Series.

The first volume of the Dark Tower series, The Gunslinger, drew inspiration from Robert Browning’s poem “Childe Roland to the Dark Tower Came” and King even borrowed the name for his heroic gunslinger Roland Deschain. The author first read the poem during his sophomore year at the University of Maine, and it stuck with him.

King explained his fascination with the poem in a 1989 interview with the Castle Rock News: “Browning never says what that tower is, but it’s based on an even older tradition about Childe Roland that’s lost in antiquity. Nobody knows who wrote it, and nobody knows what the Dark Tower is. So I started off wondering: What is this tower? What does it mean? And I decided that everybody keeps a Dark Tower in their heart that they want to find.”

2. An odd ream of paper helped, too.

King writes in an afterward to The Gunslinger, “The Dark Tower began, I think, because I inherited a ream of paper in the spring semester of my senior year of college … The ream of paper I inherited was bright green, nearly as thick as cardboard, and of an extremely eccentric size—about 7 inches wide by 10 inches long, as I recall.” In need of a project to fill out this strange green paper, King began writing the first book in March 1970.

3. T.S. Eliot’s work also makes an appearance.

Browning isn’t the only famous poet who influenced King. The series’ third installment, The Waste Lands, nearly duplicates the title of T.S. Eliot’s The Waste Land. The two main sections of the novel (“Jake: Fear in a Handful of Dust” and “Lud: A Heap of Broken Images”) directly allude to lines from the poem.

4. The series wasn’t an immediate hit.

The Gunslinger came out in a limited hardcover edition in 1982, but the first mass-market edition didn’t drop until 1988. King explained the publication delay in the 1989 Castle Rock News interview:

There were really two reasons. One-was I didn’t think anybody would want to read it. It wasn’t like the other books. The first volume didn’t have any firm grounding in our world, in reality; it was more like a Tolkien fantasy of some other world. The other reason was that it wasn’t done; it wasn’t complete ... [I]t made a certain amount of sense, but there was all this stuff that I wasn’t talking about that went on before the book opens, and when the book ends, there’s all this stuff to be resolved, including: What is this all about? What is this tower? Why does this guy need to get there?

5. Keep your eyes open for Harry Potter references.

King also paid homage to more contemporary fantasy works. In Wolves of the Calla, the author uses the same font for his chapter titles as the ones used in all seven Harry Potter books. The titular wolves use golden homing grenade-like weapons called “sneetches” (a few letters removed from everyone’s favorite Quidditch ball) stamped with a familiar-looking serial number: 465-11-AA HPJKR. The “HPJKR,” of course, stands for “Harry Potter, J.K. Rowling.”

6. King may someday write himself out of the books.

Although he’s famous for making cameos in movies and TV miniseries based on his novels, King had second thoughts after including himself as a character in the series. The author mentioned in an interview with fellow scribe Neil Gaiman for The Sunday Times that he would consider writing out the author proxy who appears in the fifth and sixth Dark Tower volumes.

7. There’s a bit of spaghetti Western in Roland, too.

While Roland Deschain takes his name and purpose from his kindred spirit in Browning’s poem, Clint Eastwood’s performance as director Sergio Leone’s spaghetti Western character “The Man With No Name” influenced the character’s look and mannerisms. The Stephen King character even winks at the comparison in Song of Susannah, telling Roland, “As The Man With No Name—a fantasy version of Clint Eastwood—you were okay. A lot of fun to partner up with.”

8. Yul Brynner is in there as well.

Wolves of the Calla tips its cap to another famous Western—this time, King shows his adoration for The Magnificent Seven. On the ever-winding path to the Dark Tower, there’s a town named Calla Bryn Sturgis: That’s Bryn as in Yul Brynner, of Magnificent Seven fame, and Sturgis as in John Sturges, the film’s director.

9. Not even King knew how it would all end.

King’s slow progress on the series had a tendency to drive fans crazy, and some tried to get the author to reveal where the series was headed. In a foreword to the fourth installment, Wizard and Glass, King wrote that an elderly cancer patient and a fan on death row had both written letters asking for the end. The inmate pledged that he would take the secret to his grave, an offer that King said “gave me the creeps.”

Unfortunately, King didn’t know how the series would end. He writes in the foreword, “I would have given both of these folks what they wanted—a summary of Roland’s further adventures—if I could have done, but alas, I couldn’t…To know, I have to write. I once had an outline, but I lost it along the way.”

10. Roland’s universe permeates all of King’s work

In an afterword to Wizard and Glass, King cemented the notion that the universe he wove in the Dark Tower series includes his other works, stating that: “I have written enough novels and short stories to fill a solar system of the imagination, but Roland's story is my Jupiter—a planet that dwarfs all the others . . . a place of strange atmosphere, crazy landscape, and savage gravitational pull…I am coming to understand that Roland's world (or worlds) actually contains all the others of my making.” King’s official website even includes a list of user-submitted connections between Roland’s story and his other novels.

11. King also borrowed characters from his previous books.

One of the series’ main characters, Father Callahan, first appeared in King’s 1975 vampire novel, ’Salem’s Lot. In his reappearance in the Dark Tower series, the priest discovers a copy of ’Salem’s Lot in a Manhattan bookstore. Other Dark Tower characters to appear in multiple King works: Randall Flagg (The Stand and The Eyes of the Dragon), Patrick Danville (Insomnia), the Crimson King (Insomnia), and Ted Brautigan (Hearts in Atlantis).

12. There may still be more to come.

When Rolling Stone asked King in October 2014 if he was done writing Dark Tower books, the writer gave a cryptic answer: “I'm never done with The Dark Tower.The thing about The Dark Tower is that those books were never edited, so I look at them as first drafts. And by the time I got to the fifth or sixth book, I'm thinking to myself, ‘This is really all one novel.’ It drives me crazy. The thing is to try to find the time to rewrite them. There's a missing element—a big battle at a place called Jericho Hill. And that whole thing should be written, and I've thought about it several times, and I don't know how to get into it.”