Bedsheet Ladder: Peter manages to make one out of threads plucked from the embroidered napkins given to him daily and woven together on the tiny but functional loom in the dollhouse he requested to keep in his cell over the span of several years.
In The Dark Tower series, Roland mentions that he met Thomas and his steward Dennis during their pursuit of Flagg after the end of the book.
Chekhov's Gun: Many. The peephole in the dragon skull that Flagg shows Thomas, where Thomas sees Flagg poison his father and Dennis finds out, years later, that Peter was innocent, bringing about Flagg's downfall. Peter's need for a napkin at every meal and his mother's old dollhouse are this as well.
Chekhov's Skill: Archery is mentioned as being Thomas's best skill, the one thing he is better at then Peter. He gets a chance to demonstrate it when he puts an arrow through Flagg's eye.
Covers Always Lie: Some later editions tried to market the book as being a horror novel. One such cover's tagline is "Once upon a time, there was a terror..."
Death by Childbirth: Sasha, while giving birth to Thomas. At least, that's what Flagg set it up to look like.
Drowning My Sorrows: As a king, Thomas becomes an alcoholic because he knows that Flagg poisoned Roland and framed Peter for it.
Love Potion: Roland doesn't really like sex, so he usually drinks an aphrodisiac potion made by Flagg before having it (at first, because he needs to produce a heir, later because he wants to pleasure his wife).
Maternal Death? Blame the Child: Thomas imagines that not only his father, but pretty much everyone in the kingdom is thinking, "We lost your mother, and we got you instead?"
Old Man Marrying A Child: A rare positive example: Roland is fifty when he marries the seventeen-year-old Sasha, but the two seem quite happy together.
Parental Favoritism: Roland favors Peter over Thomas, interestingly because Thomas is just like him; not very smart and unattractive.
Perfectly Arranged Marriage: Flagg persuaded the king to marry Sasha because he believed that she, a very minor noble who was little more than a child at the time of the marriage, would be weak and easily manipulated. He was proven wrong when Sasha turned out to be intelligent and clever, and the king genuinely came to love her so much that Flagg backed out of a murder attempt on her because he felt the king's love for her was so great that he would not have rested until he uncovered her murderer. Although it didn't stop him from eventually figuring out a way to discreetly dispose of Sasha.
Sequel Hook: The novel ends with the statement that Dennis and Thomas eventually found and confronted Flagg, but that it is a tale for another day. The Drawing of the Three makes reference to this, but almost three decades after the novel's initial publication the plotline is still dangling.
Sibling Yin-Yang: Peter is essentially the golden child, being handsome, intelligent, multitalented and beloved by all, whereas Thomas is a meek boy who feels overshadowed by his older brother, his only talent being archery.