Passage: The units of Age of Empires III are based, as in previous iterations of the game, around military classes of the historic time period. The player controls a variety of civilian and military units, and uses them to expand and develop their civilization, as well as wage war against opponents. The base unit of a game is the settler, responsible for gathering resources and constructing buildings, in order to improve the economy of the civilization. The number of units a player can control in a scenario is limited by a "population limit", a common real-time strategy game mechanic. Houses and town centers raise the starting limit, to a maximum of 200, while each unit that is produced increases the population count. Basic units such as settlers and infantry count as 1, but others, including most cavalry and mercenary infantry count as 2. More powerful units, especially artillery or mercenary cavalry, can count for a population as high as 7. Native warriors, explorers, tamed and grazing animals, hot air balloons and warships do not count towards the population limit, but generally have a build limit, allowing the player to deploy only a certain number of those specific units at a time.

Question: What was united ages of empire iii based
around military classes