The Spanish monarchy can be said to be founded by the dynastic marriage between Isabella I of Castile and Ferdinand II of Aragon, which united Spain in the 15th century. From then onwards, the Iberic peninsula was no longer an area ruled by several kingdoms, but it became a unified country, and a major political power in Europe and the rest of the world.
Through colonization, the Spanish territories comprised all continents and the Spanish King ruled a territory where the sun never set. As centuries passed, the Spanish lost control over their colonies, but within Europe they retained their important political role. The Spanish monarchy continues to this day, only deposed for a few decades when Spain came under the rule of General Franco. Yet the bloodline was continued, and the King who sits on the Spanish throne today is a legitimate descendent of the long line of Spanish Kings and Queens that ruled before the 20th century.