The Dark Tower is a series of eight books which span an entire connected universe which links several other books and stories by Stephen King, including Bag of Bones, The Talisman, Black House, The Stand, Everything's Eventual, From a Buick 8, Hearts in Atlantis, Insomnia, The Eyes of the Dragon and 'Salem's Lot, with minor references to It and The Mist.
The Dark Tower is 600 feet tall, stands in the eastern land known as End World, is surrounded by a field called Can'Ka No Rey, and is held together by six beams that stretch to the corners of the land and are guarded by twelve massive beasts on each end.
When Abbey Lee entered talks to appear as the "female lead" of Tirana, it caused some confusion among fans of the book series, as the character was originally written as a minor villain with little influence on the plot and does not appear until the final book in the series.
Idris Elba appeared the previous year in Star Trek Beyond (2016) opposite the late Anton Yelchin. Yelchin had appeared in Hearts in Atlantis (2001), which was also based on a Stephen King novel with connections to the Dark Tower series: Anthony Hopkins's character appears in the final novel of the series, and notes that Jake Chambers could easily have been a "twinner" for Bobby Garfield. Elba also appeared opposite Hopkins in Thor (2011).
The trivia items below may give away important plot points.
Stephen King's 'Dark Tower' novels connect many other novels into a single fictional universe. Walter O'Dim, AKA The Man in Black, is later revealed to use the alias Randall Flagg (Jamey Sheridan), under which guise he caused the events of The Stand (1994). The fourth novel, Wizard and Glass, involves Roland and his ka-tet traveling to the world in which The Stand takes place. Father Callahan, the Catholic Priest from Salem's Lot (1979) appears in the last three books in the series. The last book features appearances by Ted Brautigan, the character played by Anthony Hopkins in Hearts in Atlantis (2001), and Dinky Earnshaw, the character played by Michael Flores in Everything's Eventual (2009). Another character from the final book, Patrick Danville, first appears as a toddler in the novel Insomnia, which has not been filmed. The alternate universe in which the Dark Tower novels take place is also the setting of The Talisman and its sequel Black House.
The movie adaptation has recently been revealed to be a sequel to The Dark Tower book series. The movie will begin where the books technically end, now with Roland beginning his quest once again, only this time with the 'Horn of Eld' in his gunna.
In The Mist (2007) a poster of a fictional movie based on The Dark Tower can be seen in the house of David Drayton (Thomas Jane). It appears when Drayton is doing a picture of Roland Deschain, played in this movie by Idris Elba. It should be a clue for an imminent movie about The Dark Tower, but the failure of the film delayed the project during several years.