The title "King of Castile" refers to the monarchs who ruled the historical region of Castile in Spain. This title emerged in the 11th century and became significant as Castile expanded its territory and influence. The kings played a crucial role in the unification of Spain, particularly during the Reconquista, which aimed to reclaim the Iberian Peninsula from Muslim rule. One of the most notable kings was Ferdinand III, who was instrumental in the conquest of Seville and the establishment of Castile as a powerful kingdom. The title eventually merged with the crowns of Aragon and Navarre, leading to the formation of a unified Spanish monarchy under the Catholic Monarchs, Ferdinand II of Aragon and Isabella I of Castile.