The Horse and His Boy is set partly in the fictional country of Calormen.
Calormen lies south of Narnia and is presented as a large, highly organized empire with its own culture, stories, religion, and political hierarchy. The novel follows Shasta and Aravis as they travel northward from Calormen toward Narnia.
Archenland is another country in the Narnian world, positioned between Calormen and Narnia. Narnia itself is the central kingdom of the series, while Telmar becomes especially important in Prince Caspian.
The book is distinctive because its main human characters begin outside Narnia. Even so, the journey connects Calormen, Archenland, and Narnia and eventually brings the story into the broader history of the Chronicles of Narnia.