The title "Duke of Castile" refers to a noble rank in the historical region of Castile, which is part of modern-day Spain. The title was often held by members of the Spanish royal family and was significant during the Middle Ages, particularly as Castile played a crucial role in the unification of Spain. The title is associated with various historical figures, including King Ferdinand II of Aragon and Queen Isabella I of Castile, who were instrumental in the Spanish Reconquista and the establishment of a unified Spanish kingdom. The title has since become less prominent, reflecting changes in the Spanish nobility and governance.