Conversation so far:
Was Isabella I of Castile married?
Isabella I of Castile marriage to Ferdinand II in 1469 became the basis for the de facto unification of Spain.
Where was Queen Isabella's empire located?
Isabella I of Castile controlled Spain, a huge overseas territory in the New World, the Asian archipelago of the Philippines, and territories in Europe, Africa and Oceania.
Did Queen Isabella and King Ferdinand II have any kids?
Isabella I of Castile and King Ferdinand II had for daughters and one son.

Question: Who ended up taking over?
Self-contained rewrite: Who ended up taking over after Queen Isabella?