Final answer:
The kingdoms of Castile and Aragon were a double monarchy because they shared a single monarch, Ferdinand and Isabella, but maintained separate governments and laws following their marriage in 1492.
Step-by-step explanation:
The kingdoms of Castile and Aragon are described as a double monarchy because they shared a single monarch while maintaining their separate governments, laws, and systems of taxation. This happened after the marriage of Ferdinand and Isabella in 1492, where each ruler was powerful in their own right—Ferdinand as the King of Aragon and Isabella as the Queen of Castile.
Despite their personal union of crowns, the two kingdoms functioned quite independently of one another but were united under the rule of the same monarchs, which is the defining characteristic of a double monarchy.