This is a broad category for monarchs who ruled part or all of what is now the country called Spain.
Modern humans first arrived in the Iberian Peninsula around 35,000 years ago. It came under Roman rule around 200 BCE, after which the region was named Hispania. In the Middle Ages, the area was conquered by Germanic tribes and later by the Moors. The foundations for the unification of the country came the 15th century, following the marriage of Ferdinand II of Aragon and Isabella I of Castile, known to history as the "Catholic Monarchs" and the completion of the centuries-long reconquest, or "Reconquista", of the peninsula from the Moors in 1492. The final unification is generally marked with the reign of Carlos I, who, in 1516, ascended to the thrones of nearly all of the important kingdoms of Spain.
This category should be limited to those monarchs who ruled in Spain prior to 1516, including Moorish rulers. As Harry Turtledove usually writes about Spain after its unification, most monarchs can be found in the category Monarchs of Spain.