The Kingdom of Castile was a medieval kingdom in the northern part of the Iberian Peninsula, which played a significant role in the formation of modern Spain. Established in the 9th century, it grew in power and territory through conquests and alliances, eventually becoming one of the largest kingdoms in the region. In the late 15th century, the Catholic Monarchs, Isabella I of Castile and Ferdinand II of Aragon, united their crowns, leading to the unification of Spain. The kingdom was instrumental in the Reconquista, the effort to reclaim territory from the Moors, and it also sponsored Christopher Columbus's voyages to the New World.