The actual historical analogues of Castille are Galicia, Leon, Navarra, and (surprise!) Castile. Like all else in the 7th Sea world, the history of Spain has been simplified to reduce the hideous complexity of the past. I've yet to find any good maps of the layout of the pazos.
This brings me to a second point about 7th Sea—namely, a lot of information is missing from the books. For example, I've never seen a list of prices for cargo anywhere, nor for the food required to feed crew members on a ship. The governments of the various nations and their nobility are only vaguely elaborated even in the kingdom sourcebooks. It's unclear whether or not the Castillian title "Don," for instance, refers only to the governors of the huge Ranchos or to all landed nobility—or indeed if there even are lesser nobility in that sense, as the Castille sourcebook refers to gubenadores and caballeros but never any mention of those running individual towns and estates.
I've simply decided to call them "Don" as well.