benefits. Terrain improvements are built by Worker units. Irrigation increases food, mines increase production, and roads increase commerce and reduce movement costs for all allied land units using them. Two civilizations must have Right of passage treaty signed to benefit from each other's roads. Buildings enhance a city in some way and cost maintenance. Like units and Wonders, each one can only be built when the requisite technology has been acquired. Buildings require financial maintenance each turn, and can be destroyed. Only one of each type of building can be constructed in each city. As in previous installments of Civilization, there are unique Wonders of the World that can only be built once per game. Wonders provide a variety of major benefits to a specific city, all cities on a continent, or to an entire empire. Civilization III also added Small Wonders, which are functionally equivalent to Wonders except that each one can be constructed once per civilization, as opposed to once in every whole game. Small Wonders have, for the most part, a sociological requirement to construct them, as well as a technological requirement. When a civilization captures a city with a Small Wonder, it is automatically destroyed. Some examples of small wonders are Wall Street, the Forbidden Palace, and The Pentagon. One of the major features of gameplay is scientific research. Completing the research of a new technology will make available new units, city improvements, and wonders of the world, as well as special bonuses and abilities that are related to the technology. The technology tree is divided into four ages (Ancient Age, Middle Ages, Industrial Age, and Modern Age); each age requires the research of specific technologies to advance to that age. Additionally, there are non-requisite technologies that nevertheless provide useful bonuses that are often essential for good empire management, or allow a civilization to install a new government. Technologies can also be traded to and from other civilizations in return for gold, resources, technologies, workers, and cities. Technologies acquired in this way can in turn be exchanged (also called 'technology brokering') for other new technologies by contacting one or more other civilizations. Citizens are the people who work in a city. There are four kinds: Laborers, Entertainers, Tax Collectors, and Scientists. If there are more citizens in a city than available tiles to work, the extra citizens automatically become Entertainers. The second expansion, Conquests, adds two