The Crown of Castile was a significant medieval kingdom in the Iberian Peninsula, which later became part of modern-day Spain. Established in the 11th century, it played a crucial role in the Reconquista, the effort to reclaim territory from the Moors. The crown's influence expanded through conquests and strategic marriages, notably with the union of Isabella I of Castile and Ferdinand II of Aragon. This union in 1469 led to the formation of a unified Spanish kingdom, paving the way for the rise of Spain as a major European power. The Crown of Castile was instrumental in the exploration of the New World, with figures like Christopher Columbus sailing under its banner.