In 1492 Ferdinand of Aragon and Isabella of Castile consolidated their power in Spain by overthrowing the Moors. The event that cemented that victory [the ultimate goal of the 2 monarchs, married since 1469, whose efforts thus revived the Reconquista (Spanish and Portuguese, 'reconquest'), which was the series of wars fought by Christian kingdoms since the 8th century in order to regain control of the Iberian Peninsula] was the conquest of the last Moorish stronghold, Granada, in 1491. The treaty with Boabdil, the sultan of Granada, marked the end of the 10-year war and the triumph of Christianity and of Ferdinand and Isabella themselves.
That sort of joint success was part of the vision the Catholic Monarchs had for the newly-liberated Spain, as they next concentrated their efforts on spiritually unifying it, mainly via allowing the Inquisition to root out what they deemed heretical elements [eg: the Jewish and Muslim subjects who refused to either convert to Christianity, or leave the Peninsula].
Another important component of that vision was spearheading the discovery of the New World: by sponsoring Columbus' expedition to the Americas in 1492, Ferdinand and Isabella laid the groundwork for the Spanish colonial empire - the resulting power and wealth that gradually began to accumulate benefitted their rule; in turn, that sort of prestige facilitated the dynastic link between the Spanish crown and the House of Habsburg, which would produce Charles V of the Holy Roman Empire, perhaps the figure most associated with Spain's Golden Age.
