The most important factor in the rise of spain as a major european power was the:

The unification of kingdoms of Aragon and Castilla. The Kindom of Castilla owes its name to the great castles that was built in the area and it controlled the Iberian peninsula while the Aragon controlled the areas in Barcelona,and was a confederate.  the unification of these kingdoms through the marriage of John and Isabella was the beginning of the glory of Spain in the medieval ages.