The Kingdom of Castile and the Kingdom of Aragon were two prominent medieval kingdoms in the Iberian Peninsula, which is now part of modern-day Spain. Castile emerged in the 9th century and became known for its military strength and territorial expansion, while Aragon, founded in the 11th century, was notable for its strategic alliances and maritime power in the Mediterranean. In the late 15th century, the marriage of Isabella I of Castile and Ferdinand II of Aragon united these kingdoms, leading to the formation of a more centralized Spanish state. This union played a crucial role in the Reconquista, the effort to reclaim the Iberian Peninsula from Muslim rule.