History
Named after the myriads of castles in the region, the Kingdom of Castile was perhaps the most prosperous nation on the Iberian Peninsula, controlling over a quarter of Spain. Originally a small district at the north of present-day Burgos, Castile slowly expanded southward, even going as far to the very north of Al-Andalus. At its very peak, Castile, along with a unified Aragon were the main combatants that took over Granada and finished the Reconquista.
Geography
Castile, which was split into Old and New Castile, had a very large area and thus was sparsely populated. Castile included the plateaus now in Central Spain, along with Sierra de Guadarrama, a mountain range that formed a natural barrier between Old and New Castile. The Castilian regions also had multiple rivers, such as the Duero, Guadiana and Tagus rivers.
Early History and Origins
Castile originated as a march - which is a political entity that serves as a neutral territory between two or more states, created by the Kingdom of Asturias. This is the reason for the many castles and towers of the region.
In the 9th century, the County of (Old) Castile, as it was known then, was in control by the Kingdom of León, with Burgos as its capital. Due to multiple invasion attempts by the Emirate of Cordoba, Castile was well defended.
However, by the 10th century, nobles such as Fernán González managed to wrest control from León, securing Castile autonomy. Although, for a short time, Sancho III of Pamplona, due to complex political and dynastic reasons, managed to acquire Castile to his son Ferdinand I in 1027, and then Ferdinand I went to become King of León and Castile in 1037.
Union with León
After the death of Ferdinand I in 1065, his territiories were divided to his children: Sancho II gained Castile, Alfonso VI gained León, García gained Galicia, and his daughters, Urraca and Elvira gained the towns of Zamora and Tora respectively.
Sancho II then proceeded to backstab his third brother by allying with Alfonso VI to conquer Galicia and divide it upon themselves. Later, Sancho II attacked León and succeded (with the help of El Cid), exiling Alfonso VI. Urraca, however, allowed the Leónese army into Zamora.
As a result, Sancho II laid siege on Zamora, but during 1072, Sancho II was assassinated by a Galician nobleman called Bellido Dolfos. This caused the Castilian army to retreat from Zamora, and also for Alfonso VI to return to Castile and claim the three provinces as his own.
Alfonso VI had three daughters: Elvira (who married Raymod of Toulouse), Urraca (who married Raymond of Burgundy) and Theresa (who married Henry of Burgundy). After Alfonso VI's death, Urraca (now widowed) gained control of his lands, and thereafter married Alfonso I of Aragon. However, this marriage fell apart, and after an unsuccessful attempt of Alfonso I to capture Urraca's lands, Alfonso I finally rejected Urraca.
After Urraca's death, her son, Alfonso VII (with Raymond of Burgundy), who was previously King of Galicia managed to reign over León and Castile. Alfonso VII also annex parts of Navarre and Aragón.
The Crown of Castile
After Alfonso VII's death, he divided up his territories once again. Sancho III was given Castile and Ferdinand II was given León. León and Castile were still firm rivals, until 1230 when Ferdinand III of Castile recieved León from his father (Alfonso IX). Ferdinand III already had Castile from his mother (Berenguela of Castile) in 1217. Additionally, he also took the Guadalquivir Valley and his son Alfonso X took Murcia.
Finally, this resulted in the Crown of Castile, consisting of Castile, León, and various Moor taifas such as Córdoba (gained in 1236), Murcia (gained in 1243) and Seville (gained in 1248).
Castile's reign was even more accentuated after the marriage between Isabella I of Castile and Ferdinand II of Aragon in 1469.
A few years laters, due to the death of Henry IV of Castile, the War of the Castilian Succession began. This was between Isabella I (Henry IV's half-sister) and her supporters (Aragon), and against Joanna la Beltraneja (Henry IV's daughter) and her supporters (Portugal and France). Ultimately, Isabella I won, which along with Ferdinand II's succession of Aragon led to a unification of Aragon and Castile due to the marriage between Isabella I and Ferdinand II.
This union between the two (also known as the Catholic Monarchs) achieved an important objective: they finished the Reconquista, by annexing Granada to their empire. Due to Castile and Aragon controlling the entirety of the Iberian Peninsula (besides Portugal), this led to the formation of Spain.