Well, apart from uniting Spain, completing the 700 year Reconquest by retaking Granada from the Moorish invaders in 1492 (main photo, the Patio de la Acequía in the Generalife Palace), funding the voyages of Christopher Columbus that opened up the New World, and laying the political and diplomatic foundations for the largest empire the world had ever known, they were also responsible for creating the Spanish Inquisition, and expelling from Spain all Jews and Muslims who refused to convert to the Catholic faith.
Isabel -daughter of Juan II of Castile and Isabel of Portugal- and Fernando -son of Juan II of Aragón and Juana Enriquez- married in Valladolid on the 19th October 1469 despite fierce resistance from many quarters. Isabel claimed for herself the throne of Castile in 1474 following the death of her brother, Enrique IV. The succession was disputed by Enrique's daughter, Juana, who many claimed was the illegitimate bastard of D. Beltrán de la Cueva. Juana's claim was supported by her husband, Alfonso V of Portugal, and resulted in his defeat at the battles of Toro and Albuera, after which Isabel's succession was officially recognised by the Madrigal Courts.
Fernando was named heir to his father's throne after the death of his brother Carlos. He inherited the throne of Sicily in 1468, and that of Aragón itself in 1479 upon the death of his father. Fernando supported his wife in her battle for succession, and the kingdoms of Castile and Aragón were united following the death of Fernando's father under the terms of the Segovia Treaty of 1475.
The marriage of the Catholic Monarchs, as they came to be known, is considered by many to be the catalyst for the unification and aggrandizement of Spain. Their first priority was to re-establish royal authority by creating the powerful Holy Brotherhood in 1476, which was a kind of judicial police force. They also created the Royal Council to replace the Courts, and named chief magistrates (corregidores) to control the towns and cities.
Having stamped their authority at home, Isabel and Fernando turned their attentions to reconquering Andalucía, which had been occupied by Moors from North Africa since the invasion of 711. The fall of Granada in 1492 (second photo, "The surrender of Granada" reproduction by Moreno Carbonero) completed the Reconquest and filled the royal coffers, enabling them to finance the voyages of the Genoese admiral, Christopher Columbus. The discovery of America later the same year provided the funds for further aggressive overseas expansion.
The success of the Moorish campaign and growing pressure from the queen's religious advisers led to the decision to expel from Spain all Jews, and Moors from the newly reconquered territories, who refused to convert to Christianity. Instrumental in this process was the Holy Inquisition, which was created in 1478 to identify and correct any new converts to the Catholic faith that had subsequently been tempted to return to their former religious beliefs.
The nascent empire expanded further in 1493 when Fernando achieved the restitution of Cerdagne and Rousillon from Carlos VII of France with the signing of the Treaty of Barcelona. There was further conflict with the French monarchy when Fernando conquered the kingdom of Naples in 1504. Isabel died the same year, and although she had named her husband to rule as regent for the rightful heiress to the throne, their daughter Juana (known as Juana The Mad because she suffered from schizophrenia), Fernando did not receive the support of the Castilian nobility and returned to his native Aragón. Juana's husband, Felipe The Fair of Austria, ruled in his stead, but his death in 1506 prompted Cardenal Cisneros to restore Fernando to the throne of Castile in 1507.
The final years of Fernando's reign were marked by recurring conflicts with the French monarchy in Italy, which continued up until his death in 1516, when he was succeeded by his Hapsburg nephew, Carlos I of Spain, Carlos V of Austria and future Holy Roman Emperor. A decisive factor in the success of Fernando's Mediterranean adventures was the appearance of the Castilian army in the war theatres of Europe, where it would dominate for the next century and a half. According to the wishes of both, Fernando was buried next to Isabel and their two children in the crypt of the Royal Chapel of Granada cathedral. The fourth photo shows their mausoleum in the main body of the chapel above.
Among the most enduring legacies of the reign of the Catholic Monarchs are the huge number of public works programmes they initiated, especially in Castile, where a large number of castles, churches, cathedrals, universities, hospitals and other buildings date from this period.