Etymology
* As an English and Scottish surname, from the noun castle.
* As a Catalan, Aragonese, and Jewish surname, from castell (“castle”), similar to the above.
* As a Manx surname, from Mac Asgaill (“son of Ásketill”), from Old Norse Áskell. Compare McAskill.
* As a German surname, variant of Kastel.
Compare Castel, Castillo.