It is incredible the amount of erroneous cliches that I have seen concerning the "creation" of Spain, and this thread shows some of them. As some of you have posted Spain was created as a personal union of Castille and Aragon. There was no takeover. Castille seemed to prevail because she had more population as she had more surface and it was more densely populated than Aragon (because of the Black Death hitting harder there). If Castille had not joined Aragon, the conquest of parts of Italy would have been harder if not impossible as Aragon profited from castillian troops and commanders.
Culturally Castille and Aragon were quite homogeneous. Castillian was the common language in most of the Kingdom of Aragon and even in Catalonia, Valencia and the Balearics it seems that it was quite widespread and seen as one of the languages of these kingdoms.
When Isabel and Fernando started to use the name "Spain" to designate their dominions the portuguese crown was enraged as they understood that the word Spain/Hispania also belonged to them as they were also part of the Roman Hispania. It seems that portuguese was quite similar to castillian then and most of the bigger differences were adopted in the XVII-XVIII centuries as a way of helping their independence. By the way Portugal never claimed Galicia.
Another cliché I have seen is that of a reformed Navarre more french than spanish. Navarre was one of the hispanic kingdoms that happened to have a french dinasty. Culturally they had nothing to do with France and they were fiercelly catholic.
Euskadi/the Basque Country is a nationalistic invention of the late XIX century. The basque lords looking for autonomy from both the kingdom of Pamplona/Navarre and the kingdom of Leon helped to the independence of Castille and they joined her freely. Basques settled lots of villages in Castille during the reconquista (you can see basque names in villages even near Madrid) and they had lots of privileges. They were proud of being spanish and even considered that they were more true castillians and spanish than the rest.