(kăstēl'), Span. Castilla(kästē'lyä), historic region and former kingdom, central and N Spain, traditionally divided into Old Castile and New Castile, and now divided into Castile–La Mancha and Castile-Leon. Castile is generally a vast, sparsely populated region surrounding the highly industrialized Madrid area. It includes most of the high plateau of central Spain, across which rise the rugged Sierra de Guadarrama and the Sierra de Gredos, forming a natural boundary between Old and New Castile. The upper Duero, the Tagus, and Guadiana rivers form the chief valleys etched into the plateau. The soil of Castile, ravaged by centuries of erosion, is poor, and rainfall is sparse.
The name Castile derives from the many castles built there by the Christian nobles early in the reconquest from the Moors (8th–9th cent.). Old Castile at first was a county of the kingdom of León, with Burgos its capital. Its nobles (notably Fernán González) secured virtual autonomy by the 10th cent. Sancho III of Navarre, who briefly annexed the county, made it into a kingdom for his son, Ferdinand I, in 1035.
León was first united with Castile in 1037, but complex dynastic rivalries delayed the permanent union of the two realms, which was achieved under Ferdinand III in 1230. The Castilian kings played a leading role in the fight against the Moors, from whom they wrested New Castile. They also had to struggle against the turbulent nobles and were involved in dynastic disputes that plunged the country into civil war (see Alfonso X). Peter the Cruel limited the vast privileges of the nobles, but they were permanently curbed only late in the 15th cent.
In 1479, after Isabella I had defeated the dynastic claims of Juana la Beltraneja, a personal union of Castile and Aragón was established under Isabella and her husband, Ferdinand II of Aragón. The union was confirmed with the accession (1516) of their grandson, Charles I (later Emperor Charles V), to the Spanish kingdoms. Charles suppressed the uprisings of the comuneros in 1520–21.
With the decline of Catalan and Valencia during that period, Castile became the dominant power in Spain. It was the core of the Spanish monarchy, centralized in Madrid (the capital after the 16th cent.). Its dialect became the standard literary language of Spain, and the character of its people—proud and austere—typifies the Spanish state. Latin America was largely influenced by Castilian culture.