Each city can only produce one military unit or one building at a time; any additional units or buildings are placed into a queue. The rate of construction is determined by the amount of material collected from the surrounding spaces; players can also choose to speed production by sacrificing gold or population if they adopt the required governmental policy; called civics. The player can instead specialize the city towards gathering a particular resource instead of constructing additional units or buildings. Also, in order to produce some units or buildings, certain resources must be collected within the empire and connected to the empires trade network by roads or harbours (for example, horses are needed for mounted units, and iron for swords, while stone or marble increase production of certain Wonders.).