The Kingdom of Castile was a historical 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 medieval Europe. In 1469, the marriage of Isabella I of Castile and Ferdinand II of Aragon united their kingdoms, leading to the creation of a unified Spain. The Kingdom of Castile was instrumental in the Spanish Reconquista, the exploration of the New World, and the establishment of Spanish as a global language.