So as we all know, the royal marriage of Queen Isabella I of Castile and King Ferdinand II of Aragon led to the unification of Spain. As one might expect, Castile and Aragon became relatively equal partners in the new realm. Aragon's Mediterranean holdings were counterbalanced by Castile's extra population.
But what if it had turned out differently? What if the two kingdoms didn't have roughly equal power, and instead Aragon, with its colonial holdings in Italy, more urbanized territory, and wealthier nobility managed to dominate? (I imagine Castile dominating Aragon is just ASB, like Russia dominating Poland).