Nota Bene: This writeup is intended as a counterpart to my writeup on Aragon, specifically to summarize the history of the other Spanish kingdom that arose during the Reconquista. It is only a history of the kingdom and not the region in general. It won't contain current or prehistorical information about the region because that more properly belongs in writeups on Castilla-la-Mancha and Castilla y León, the present political designations.
Castile, Castilla in Spanish, was one of the Christian kingdoms that emerged from the territory reconquered from Spain's Moorish occupants. After the North African Moors conquered Spain between 711-718, Spain's Christian populace slowly, over a period of nearly 800 years, retook Spain starting with its mountainous northern coast.
The first Christian kingdom in Spain after the beginning of the Reconquest was that of Asturias, located on Spain's northern coast. Asturias, the only part of Spain never held by the Moors, slowly grew as more and more territory was taken back by Christians. Soon it grew to encompass the city of León, which became the capital of the kingdom of León during the 10th century. The kingdom of León dominated northwestern Spain for hundreds of years.
The expansion of Christian Spain was slow, and new territory was added gradually, and at first Castile was a county of León. Castile, Castilla in Spanish for the numerous castles that dot its landscape, was the object of struggle for centuries, heavily fortified to this day, evidence of the battles fought over it.
Soon Castile became a kingdom in its own right, first during the ninth century, although not in name, and as a matter of law in 1035, when it was made independent by Sancho III of Navarre (who had annexed the territory), and given to his son Ferdinand I. The Reconquista had become a matter not of war but of the Santa Fe, the Holy Faith, as the fight against the Muslims, who had peaceably granted Christians the right to practice their religion, assumed the stature of a Holy War of Christ against the infidels, and the stage was set for The Spanish Inquisition, although it was still hundreds of years off.
This was the era of the Crusades, and all Christendom was moving to fight the Mohammadans who threatened the supremacy of Christ. Ironically, Spain was the conduit for a great deal of learning about medicine, science, and mathematics that Arabic scholars had achieved. Castile, for most of this period, was the supreme power of Spain, opposed by Aragon and Portugal, but Castile was the center of the reconquest, Slowly it grew, wresting New Castile from the Moors, and the kingdom grew to encompass most of the Iberian Peninsula.
León was permanently united with Castile under Ferdinand III in 1230. His reign also marked the end of the dynastic wars that had been fought in Castile, which had sapped its power among the other kingdoms on the peninsula. Castile had its capital in Toledo and fought a reunified Moorish empire, a province of a North African kingdom, but this unity was short lived, and Islamic Spain broke down into separate emirates, leaving southern Spain open to Christian conquest. During this period, Córdoba (1236) and Sevilla (1248) were retaken by Castilian armies.
By the latter 13th century, the Moors were consigned to a small area around Granada and the war against them was no longer a force encouraging Spanish unity. This was toward the end of the medieval period, and the Spains faced the same decline that troubled all of Europe's governments, and conflict both within and between the various kingdoms was the norm in Iberia. Castile's ruler from 1252-1284 was Alfonso X, the Wise, a scholar who encouraged poetry, music, and art, and promoted Castilian as a literary language. Schools were established throughout Castile, but Alfonso X's political accomplishments didn't display the same wisdom.
Alfonso fought for election to the position of Holy Roman Emperor, spending incredible sums and ultimately facing humiliation as, after being elected by a narrow majority, his appointment was blocked by the Pope. The nobility of Castile took advantage of his vulnerability and the monarchy was weakened greatly, leading to his deposition in 1284.
An important event occured in 1375, when John, a Castilian prince and son of King Enrique II married Eleanor, daughter of Aragon's King Peter IV. The two kingdoms began a period of close relations, and their governments started to institute similar policies, loosening their rule and allowing greater regional independence, important at a time when each kingdom had grown to encompass several other previously independent states. Castile began to develop its navy, which would later become the Spanish Armada, and it pursued trade in the Atlantic with Great Britain, France, and Northern Europe. Aragon became a great trading power in the Mediterranean and held territory around much of the perimeter of the sea.
In 1469, Ferdinand of Aragon and Isabella I of Castile were married, and with the marriage of these two monarchs, the two great kingdoms were joined, and ruled jointly. The Spain's became a single body, and in the ensuing years were Europe's supreme power. All of Spain was wrested from Moorish control with the defeat of Granada in 1492, a momentous year for Spain that included as well the expulsion of Spain's Jewish and Muslim populations and the establishment of Spain in the New World. Castile assumed cultural supremacy in Spain, its tongue becoming the national language and its writings the national literature.
After this point, a history of Castile is a history of Spain. Today two of Spain's autonomous communities include portions of Spain: Castilla y León (Old Castile) and Castilla-la-Mancha (New Castile).