Chapter 1: The Gunslinger
In the first chapter of The Gunslinger, Roland tells some of his story to Brown. In his story he recalls the events that took place in Tull that lead him to kill all the towns people (thirty nine men, fourteen women and five children for a total of fifty eight citizens).
Roland is then forced to kill his lover, Allie, and while the town pursues Roland (trying to kill him) he ends up killing them all.
The Battle of Tull
In the Marvel season of The Dark Tower entitled "The Battle of Tull" it is based on the same events mentioned in The Gunslinger.