Castle in the Air is a sequel to Howl's Moving Castle (1986), also a romantic fantasy. Characters who reappear in Castle in the Air include Howl, Sophie, and fire demon Calcifer. Part of the novel's fun is finally learning what enchanted parts they played. The moving castle stolen by the djinns Dalzel and Hasruel belongs to Howl.

The land of Ingary appears in both novels, as do magic spells and wizards.

In Howl's Moving Castle, the relationship between Howl and Sophie is explored. Sophie is a charming young hatter until a wicked witch turns her into a bossy old woman. She seeks shelter as a housekeeper to Howl, a young wizard who dwells in a moving castle. Each tries to undo magic while becoming involved in a blossoming romance.

Unlike Flower-in-the-Night, who is assertive but remote from Abdullah for much of his tale, Sophie has an important...