"Age of Empires III", released on October 18, 2005, was built on an improved version of the "Age of Mythology" game engine with the most significant changes being the updated graphics engine and the inclusion of the Havok physics middleware engine. The game is set in the period between 1421 and 1850, and players can choose one of eight European nations. The game introduced a large number of features, such as home cities. Described by Ensemble Studios as "an important support system to your efforts in the New World," home cities help provide the player with resources, equipment, troops, and upgrades. They can be used across multiple games, and upgraded after each battle; the feature was compared to a role-playing game character by Ensemble Studios. The first expansion to "Age of Empires III", "The WarChiefs", was released October 17, 2006. Most gameplay changes in the expansion pack were small, but it introduced three new civilizations, with a focus on Native Americans. Most notable was the introduction of the unit. The second expansion, ', went on sale October 23, 2007. It was a jointly developed product; Big Huge Games helped Ensemble Studios develop the game, with Brian Reynolds joining Bruce Shelley as lead designer. The game expanded the "Age of Empires III" universe into Asia, and introduced three new civilizations. Reception towards "Age of Empires III" was mixed; Game Revolution described it as "about as much fun" as a history textbook, while GameZone argued it was "one of the best looking games, much less an RTS game, that is out on the market currently". It sold more than two million copies, and won the GameSpy "real-time strategy game of the year" award. "The WarChiefs" failed to equal the success of its predecessor, with a lower score on both Game Rankings and Metacritic, and "The Asian Dynasties score was lower still with 80%.