Religion
Medieval Total War
Rome: Total War: Barbarian Invasion
In Rome: Total War, factions all could build monuments to their gods, which would only affect the happiness of a settlement. In Barbarian Invasion, religion has been reworked entirely.
By 363 AD, religion had become extremely important in people's lives. In the early days of the Empire, religions could quite easily exist alongside each other (The Persian god Mithras becoming a famous god amongst the Romans is just one example), but it is no longer so.
Religion now affects your settlements' happiness in the largest way possible. Building a Chritsian church in a largely Pagan province now will result in extreme unrest and possibly rebellion. The same is likewise if you place a pagan temple in a Christian town can cause rebellion among its people. Many barbarian factions will try to appease their Roman subjects by adopting their religion. (i.e. the Franks become Catholics, to gain the support of the Roman Gauls.)
