Religion
Religion has been a factor in most Total War games so far. However, its importance has changed over time. Whereas it seriously affects the gameplay in some games of the series, it's a minor factor which barely matters in others.
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Medieval Total War
Rome: Total War
In Rome: Total War religion plays a rather minor role. The factions can build various temples or shrines to different gods. These temples grant bonuses which are in accordance with the role of the respective god. Whereas gods of war tend to give military based bonuses, other gods or goddesses can for example make a region's farms more effective or will improve the health and happiness of the local citizens.
Rome: Total War: Barbarian Invasion
In Rome: Total War, all factions all can build monuments to their gods which only affect the settlement. However, in the Barbarian Invasion expansion religion has been reworked entirely.
Religion has become extremely important in people's lives. In the early days of the Empire, religions could quite easily co-exist peacefully but times have changed and religious unrest is now very common.
There are three religions; Christianity, Paganism and Zoroastrianism. Most characters and nearly all generals have a religious belief. Religious conversion rates in a given region are effected by characters and buildings in the given region, and by the official religion of neighboring regions.
Each settlement has an Official Religion, and each faction has a State Religion. The State Religion is determined by the faction leaders religion (check traits and ancillaries). The Settlement Official Religion is determined by buildings present, or by the governor if there is no building, or by faction leader if there are neither. Any settlement that has a different Official Religion than the State Religion gets 10% unrest. The faction leaders religious trait has a huge effect on what religion new characters follow; for example if your faction leader has the Christian trait, nearly all new characters will have the Christian trait as well. For this reason the player should consider religion carefully when deciding who to make the faction heir. If you want to change your State Religion from one to another, you have to find a family member with the Religious trait you want, and then make him the heir, then the leader. This can take a long time for the non-Roman factions.
Religion affects a settlement's happiness considerably. Building a Christian church in a mainly Pagan province will result in considerable unrest and possibly even rebellion. Placing a Pagan temple in a Christian town can cause the same problems. Religious unrest can go as high as 100%, and this combined with other factors can make certain provinces impossible to hold. In order to appease their subjects, most factions can decide to build religious buildings of the religion which is prevalent in a region - even if the faction's State Religion is a different one.
Having a governor in a city with a different religion from the Official Religion causes 5% unrest, and can cause negative traits, so it is not ideal. Generals can never properly change their Religion trait without modifications to the game.
The Berbers are purely Christian, the Huns and Saxons are purely Pagan, the Sassanids are purely Zoroastrian. These factions will only spawn characters of their religion and cannot build other religious buildings. All other factions can choose to be Christian or Pagan. Generally speaking, Christianity provides better public order, law, and conversion traits and ancillaries while Paganism provides more military bonuses and its buildings need less investment. Some factions also have certain units linked to religion, such as the Christian Frankish Paladins and the Pagan Lombard Berserkers.
Medieval II: Total War
Empire: Total War
Religion plays a rather minor role in Empire: Total War. Regions that are of a different religion to the faction that controls them will experience happiness penalties due to religious unrest. Factions may choose to build religious centres in Towns, which gradually begin to convert the local populace to the faction's religion. Religious centres grant a moderate happiness boost to a region that is predominantly the same religion as the owning faction; however, the happiness bonus is less than that of dedicated entertainment centres. These same centres can also spawn missionaries, who can travel into other regions to begin converting the populace to their religion. This can be used to either cause unrest in other factions' lands or to help stabilize religious unrest in your own or allied regions.
Religion effects diplomacy as well. Factions that share a religion get an increase in relations, while opposing religions gets a decrease. These can easily be overcome however, especially when the religions are similar enough. They make the biggest difference when the 2 factions are otherwise neutral.
Some religions are more compatible than others. For example if a Protestant faction rules a region that is entirely Catholic and Orthodox there will be 2 points of unrest, but an entirely Muslim region will have 10 points of unrest. The late-game enlightenment technology "Human Secularism" cuts the effects of both religious unrest and happiness by half, making religion a very minor point in the late-game.
Nations cannot change their religion, and all religions function the same way except for slight differences with Animism.
Empire: Total War: Warpath Campaign
Religion works the same way, although the Native American factions are Animist and have different buildings for conversion than all other religions. Their religious building is built in the Region Capital as opposed to outside Towns for the other religions.
Additionally, Native factions can build very cheap Pioneer buildings, which generate a lot of wealth but convert the population to Protestantism, a foreign religion that causes great unrest.
Napoleon: Total War
Religion plays a similar but even smaller role than in Empire: Total War. Regions that are of a different religion to the faction that controls them will experience happiness penalties due to religious unrest, but these penalties are usually small since the Map is overwhelmingly Christian. Religious Unrest is only really serious in the Egypt Campaign and for the Muslim Ottomans in Europe if they expand. There is no way to convert regions or to change your own religion, meaning that religious unrest will simply have to be countered in other ways, either through buildings, ministers or garrisons.
Additionally there are small diplomatic bonuses between factions of the same religion, and small penalties between factions of different religions, but these bonuses usually are dwarfed by other factors.
Napoleon: Total War: Peninsular Campaign
In the Peninsular Campaign, religion is replaced by political alignment, which can be either Pro-French or Anti-French. The effects of political unrest are much higher than in the base game. Political alignment in a given region can be changed either by Educational Buildings, or by Priests or Provocateurs who are spawned by those buildings. These agents also increase happiness when in friendly cities, as well as unhappiness when in enemy cities.
Additionally, looting a region results in a 50% swing against your factions political alignment in that region.
Shogun II: Total War
Religion can be very important in Shogun 2, if the other religions besides Shinto-Buddhism manage to take hold. In some campaigns, however, they may not take hold and in those games the player can largely ignore Religion. Each Religion has various bonuses and weaknesses, and Regions with a population of a different Religion than the ruling Clan get very significant levels of unrest.
Shinto-Buddhism is the default Religion for the vast majority of Clans. Shinto-Buddhist clans get the benefit of access to Warrior Monks, better diplomacy with the other Shinto-Buddhist Clans and usually not having to worry much about Religion, allowing quicker expansion.
Christianity is a foreign religion. It can play a major role in the later-game if it spreads and clans decide to convert. Christianity becomes available when the Nanban (Portuguese Traders) reach your clan, the further East your clan is the longer it takes. Once you come into contact you can build a Nanban Port, which provides wealth and Matchlock units. However, Christianity also spreads from a Nanban Trade Port, and it will cause severe religious unrest throughout the region if the ruling clan does not either convert or control the spread.
Christianity allows a faction to access gunpowder options earlier and allows the construction of Nanban Trade Ships, which will absolutely devastate most opposition on the seas. Christian buildings are also better at conversion than Shinto-Buddhist buildings and help research the Chi arts faster than their Buddhist counterparts. Additionally Nanban Quarters, a upgrade to the Nanban Trade Port available for Christians, generate a lot of region wealth and trade routes and help ease the conversion process. However, conversion hurts the Daimyos honor and causes the faction to have to convert their regions, which is a long process. Since the map is almost entirely Buddhist Christianity will slow down expansion, since public order will become a problem in conquered regions. Conversion also causes the faction to lose access to the very powerful Warrior Monk units; any existing Warrior Monks will disband on conversion. Christian factions also get a -40 Diplomatic penalty with Shinto-Buddhist factions, although with other Christian factions they get a massive +50 bonus.
Christianity begins with a foothold on the Southern Island of Kyushu. It often spreads further and factions in that area sometimes choose to convert since that is easier than stamping it out. The Otomo, who are on East Kyushu, start as a Christian clan and can never return to Buddhism. If Kyushu ends up dominated by a Christian Clan, you may wish to convert to Christianity simply to establish strong relations with that Clan.
The Ikko-Ikki have their own form of Buddhism and can never convert. They get a -20 Diplomatic penalty with Shinto-Buddhist factions and an enormous -80 with Christians. Their Religious buildings research the Arts even slower than the Shinto-Buddhists but convert very quickly and provide Warrior Monks as Garrisons. Ikko-Ikki Religious rebels also join the Ikko-Ikki Clan if they conquer the region.
Religion is spread from the specific Religious buildings and by Religious Agents recruited from them. These Agents can Inspire Towns, Convert enemy agents, causing them to disband, Inspire Armies, demoralize Enemy armies, and Incite Rebellion in Rival Clans Towns. Interestingly, an Incited Revolt is more likely to succeed when less of the population follows the Monks religion.
Shogun II: Rise of the Samurai
In Rise of the Samurai, Religion is replaced by Political Alignment. Every clan is tied to the Taira, the Minamoto, or the Fujiwara families. A clan can never change which family it is tied to, although it can make alliances with clans of other Political Alignments. Clans with the same Alignment get +30 to Diplomatic Relations, those with opposing get -60. Like religion in the base game, regions with a different Political Alignment to the ruling Clan get a lot of Political Unrest.
Political Alignment is spread by the Tax Office buildings, and by the Junsatsushi agent who is recruited from the Market buildings. If a region has at least 50% support of a certain family, the Junsatsushi can go into the Castle-Town and try to convince it to join his clan. A success, however, is an act of War, unless the action causes the last Town of a Minor Clan to join the Clan of the agent.
Monks can also incite rebellions in regions that are ruled by a Clan of a different Political Alignment, his chance of success is increased the more the Region supports his particular Political Alignment. Unlike in the base game, a Monk cannot incite a rebellion in regions controlled by a Clan of the same Political Alignment.
Shogun II: Fall of the Samurai
Religion is also Political Alignment in Fall of the Samurai. Every Clan is either Pro-Shogunate or Pro-Imperial, at least until Realm Divide. Clans with the same Political Alignment get a very large +75 diplomatic bonus, those with a different Political Alignment get an enormous -150. Like the in base game and Rise of the Samurai, regions with a different Political Alignment to the ruling Clan get a lot of Political Unrest. Unlike in Rise of the Samurai, a Daimyo can choose to change sides once in his lifetime as long as Clan fame is not too high.
Political Alignment is spread by Propaganda buildings, and by the Agents recruited there. These Agents can also incite Rebellions in the regions of other Clans; their chance of success is determined by how much of the population supports their cause. Unlike in Rise of the Samurai, agents can cause rebellions in Regions even if the ruling Clan shares the same Allegiance.
Once Realm Divide occurs, the player can either stick with whoever he currently supports, or declare himself Independent. If he stick with his ruler, the Emperor or the Shogun, than the player becomes his Vanguard. All Clans of the other Allegiance declare War on the player and all the Clans of the same Allegiance side become more friendly and will accept any Alliance. If the player declares himself an Independent Republic, his Clan gets it's own Political Alignment, called Independent. All Clans will then automatically declare War on the new Republic for the rest of the game. The player will also have to convert his regions again, although Republics get less Political Unrest than Imperial/Shogunate Clans. Independent Clans can not even make Alliances with Clans that emerge after Realm Divide, it is complete Total War for the whole campaign.
Ishin Shishi are the Imperial Agent, Shinsengumi are the Shogunate Agent, and they disband if their Clan changes Allegiance. Independent Republics receive no Agent at all, to compensate their Propaganda buildings are a little better at providing Public Order.