Rise of Nations employs the concept of "territory," as employed in the Civilization long series of games; the area near the player's settlements is considered their territory, and players may only construct buildings within their territory or that of an ally. A nation's borders can be expanded by the creation and expansion of cities and forts, a technology tree, and obtaining access to certain rare resources. Other technologies and resources cause enemy units to suffer attrition over time, which can eventually destroy an unsupported invasion force.rnrnCities are centrally important to game-play; most buildings can only be built within a certain distance of a city, borders are most easily expanded by building and expanding cities, and cities are the only source of the resource-collecting Citizen unit. Only a limited amount of cities can be built and cities can only be destroyed by its owner.