Isabel I of Castile, born in 1451, reigned as Queen of Castile from 1474 until her death in 1504. Her marriage to Ferdinand II of Aragon in 1469 created a dynastic partnership that laid the foundations for the unification of Spain. Together they became known as los Reyes Católicos (the Catholic Monarchs), a title formally granted by Pope Alexander VI in 1494 to honour their defence of the Catholic faith, their triumph at Granada, their enforcement of the Inquisition and their sponsorship of voyages to spread Christianity overseas.

The defining achievement of Isabel’s reign was the completion of the Reconquista, the long struggle to "reclaim" Iberian lands from Muslim rule. By the late fifteenth century, the Nasrid Kingdom of Granada remained the last Muslim stronghold. From 1482 to 1492 Isabel and Ferdinand directed a determined campaign of sieges and battles that gradually weakened Granada’s defences. Isabel travelled with the armies, supervised supplies and administration, and gave the war an air of unyielding resolve. In January 1492 the city of Granada surrendered, ending nearly eight centuries of Muslim political rule in Spain. Across Christendom the victory was celebrated as a triumph of the Catholic faith, and Isabel herself was seen as a warrior queen whose determination had brought the struggle to its end.

Victory was followed by an uncompromising drive for religious uniformity. In March 1492 Isabel and Ferdinand issued the Edict of Expulsion, forcing Spain’s Jews to convert or leave. Those who converted, known as conversos, remained under suspicion and surveillance. Muslims too came under increasing pressure. After the fall of Granada many were compelled to convert, creating a population of moriscos who were closely monitored by the authorities. The Spanish Inquisition (la Inquisición Española), authorised in 1478 but expanded under Isabel, became the chief instrument of religious policing. It investigated heresy, censored texts and instilled fear, narrowing the cultural and religious diversity of Spain.

The same year as the fall of Granada, Isabel also looked beyond the Iberian Peninsula. She agreed to sponsor the Genoese navigator Christopher Columbus, whose westward voyage in 1492 opened the way to Spain’s overseas empire. Isabel was determined that the new territories would not only enrich Spain but also serve the Catholic faith. Instructions issued in her name required that Indigenous peoples be converted to Christianity, embedding missionary work into the fabric of Spanish colonisation. Her sponsorship tied the birth of the empire to both wealth and evangelisation, a legacy that shaped Spain’s role in the wider world for centuries.

Isabel’s dynastic policy projected her influence across Europe. Her eldest daughter, also named Isabella, married the heir to Portugal but died young. Her only son, John, Prince of Asturias, was seen as the great hope for dynastic stability but died in 1497, leaving succession uncertain. Joanna, known as Juana la Loca, married Philip the Handsome of Habsburg, making Isabel the grandmother of Charles V, who would rule both Spain and the Holy Roman Empire. Maria married another Portuguese king, strengthening Iberian links. Most famously from a British perspective, her daughter Catherine of Aragon married first Arthur, Prince of Wales, and then Henry VIII, becoming Queen of England and mother of Mary I. Through these marriages Isabel tied Spain’s fortunes to the leading dynasties of Europe.

When Isabel died in 1504, she left behind a kingdom transformed. She had unified Castile and Aragon in a dynastic partnership, completed the Reconquista, enforced Catholic orthodoxy and sponsored voyages that launched Spain’s empire. Yet her reign is also remembered for intolerance: the expulsion of Jews, the persecution of Muslims and moriscos, and the establishment of the Inquisition curtailed the cultural richness of Iberia. Isabel of Castile’s reign was a decisive turning point, forging a united and powerful Catholic Spain at the cost of exclusion, repression and empire built on conquest.