Cities have a variety of buildings that may be built or upgraded, such as: temples, aqueducts—and amphitheatres, which increase the people's happiness and well-being. Markets increase the city's financial contribution, and academies the likelihood of producing effective family members (see below). Walls make the city more resistant to assault, and barracks, archery ranges and stables unlock new military units. The player expands their empire by training armies in friendly cities and using them to assault and occupy enemy cities (native mercenary units may also be hired by a family member outside a city). Controlling more cities benefits from increased geographical dominance and greater tax income. However, more cities and larger populations become increasingly difficult to control, as local populaces resist foreign rule, and reinforcements have further to travel. If a city's inhabitants are overtaxed, underdeveloped or unprotected, they rebel and become in effect their own faction - the player's control is lost, garrisoned units are forced out of the city, and a hostile rebel army is formed in its place.