The nation that was united by the marriage of Isabella and Ferdinand was the nation of Spain. Spain in the map has a light, peachy colour, and it is the first red square of the map, located at the bottom left of the image.
Ferdinand and Isabella were known as the Catholic Kings. Their marriage in 1469 united the kingdoms of Aragon and Castile, giving rise to the nation of Spain. They were also responsible for the reconquest of Granada from the Moors (1492), the discovery of the New World (1492) and the strengthening of the Spanish Church.