The climate seems to be similar to the Middle East in Our World. Culturally, the Sultanates seem inspired more by a Westernized [Arabian Nights] style mythology then by the reality of any Middle Eastern country at any point in Our World's history, much as Ingary seems more inspired by European fairy tales than by any real European country. There's no particular religion in the Sultanates, although djinn are real and have to follow a specific moral code.
Specific towns and regions that appear in the books: Zanzib
See also: The Land of Ingary