The two instances that Stephen King mentions in that answer belong, primarily, to the events of the second book in the longer The Dark Tower series, titled The Drawing of the Three. In that novel, Roland Deschain of Gilead (played in the film by Idris Elba) is attacked by lobster-like creatures on a mysterious beach. During the skirmish, Roland loses two fingers on his right hand -- an injury that plagues him for the duration of that novel. Later in the book, Roland must step through three floating doors that carry him into a version of our world in different time periods. From these worlds, he extracts people who are supposed to help him on his mission to reach the Tower. King likely had a vision of floating doors on a desolate beach, but we never got that imagery in Nikolaj Arcel's film. They looked a little bit like this in the novels: