Age of Empires is a series of historical real-time strategy video games, originally developed by Ensemble Studios and published by Xbox Game Studios. The first game was Age of Empires, released in 1997. Nine total games within the series have been released so far as of October 28, 2021. 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, Age of Empires II: The Age of Kings, was set in the Middle Ages, while its expansion focused partially on the Spanish conquest of the Aztec Empire. 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. Another 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. The fourth main installment in the series, Age of Empires IV, was released on October 28, 2021. The Age of Empires series has been a commercial success, selling over 25 million copies. Critics have credited part of the success of the series to its historical theme and fair play; the artificial intelligence (AI) players have fewer advantages than in many of the series' competitors. Games The games in the series focus on historical events throughout time. Age of Empires covers the events between the Stone Age and the Classical period, in Europe and Asia. Its expansion, The Rise of Rome, follows the formation and rise of the Roman Empire. The Age of Kings and its Nintendo DS spin-off follow Europe and Asia through the Middle Ages. The Age of Kings expansion pack, The Conquerors, is set during the same period, but also includes scenarios about the Spanish conquest of the Aztec Empire, El Cid, and Attila the Hun. Age of Empires III and its first expansion, The WarChiefs, take place during the European colonization of the Americas. Its second expansion, The Asian Dynasties, follows the rise of Asia in the same period. Age of Empires Online focuses on the Greek and Egyptian civilizations. The series' spin-off, Age of Mythology, and its expansion pack,