Spain
The most powerful monarchy in Europe in this period was Spain. In the mid-1400s, King Ferdinand of Aragon and Queen Isabella of Castille married and united their separate kingdoms to create the kingdom of Spain.
Then, they married their daughter to the son of the King of Austria who himself was married to the heir to the Duke of Burgundy (who controlled the area to the north of France, what is presently the Netherlands as well as parts of Italy). All these areas, then, descended to the grandson of Ferdinand and Isabella, who was also the grandson of the King of Austria (whose family name, Hapsburg, he kept).
This heir was Charles V, and after becoming King of Spain and of Austria, he was also elected by the German princes in 1519 as Holy Roman Emperor.