"Age of Empires" focused on events in Europe, Africa and Asia, spanning from the Stone Age to the Iron Age; the expansion game explored the formation and expansion of the Roman Empire. The sequel, "", was set in the Middle Ages, while its expansion focused partially on the Spanish conquest of Mexico. The subsequent three games of "Age of Empires III" explored the early modern period, when Europe was colonizing the Americas and several Asian nations were on the decline. The newest installment, "Age of Empires Online", takes a different approach as a free-to-play online game utilizing Games for Windows Live. A spin-off game, "Age of Mythology", was set in the same period as the original "Age of Empires", but focused on mythological elements of Greek, Egyptian, and Norse mythology. A fourth main installment in the series, "Age of Empires IV", is under development, and is planned for release on October 28, 2021.