Horde Mode - All the nomadic factions such as the Franks, Vandals and Goths, to name a few, have the ability to raise a horde of troops when their last settlement is either taken or abandoned. It is meant to represent a mass migration of a people out of the steppes of Asia looking for more prosperous lands in the Roman Empire or in the case of some factions, to look for safety from the Huns who have pushed them out of their own lands. When in horde mode the faction has no cities and no income, but has the ability to sack captured cities (similar to razing a city, but more complete and resulting in more loot). The faction either chooses to sack a city, which means they continue to move as a horde, or occupy, which means the horde settles down in that territory. The faction's armies also swell due to the whole population of their faction rising up to move. Horde-Specific units will join your armies, such as unique cavalry and infantry units. Most of these units will immediately disband and settle in the city the faction captured and decides to occupy. A faction as such because of horde mode can only be destroyed when all of the royal family members are killed.
Emergent Factions – Through the course of the game, new factions can emerge . Exampls are the Romano-British Faction, which forms if the Romans are pushed out of Britannia, and the Ostrogoths, which form when a Goth-Controlled city rebels against that faction. Roman cities can also rebel and join the larger rebel faction (Roman Western Empire Rebels or Roman Eastern Empire Rebels respectively) with whom full diplomatic actions can take place. The Slavs also appear as a new faction in 410 AD.
Night time battles – Battles can now take place during the night if the player’s general has a specific trait that allows him to do so. Using night time battles the player can use surprise attacks or restrict enemy reinforcements from entering the battle, as well as other things.
Multiple Victory conditions – Barbarian Invasion's campaigns do not have short or long campaigns. Instead, they have victory conditions specific to factions. For example, the Franks have to capture territories in Western modern day France, while the Huns have to conquer both Constantinople and Rome. Each faction has its own specific victory conditions that must be met
Religion – There are 4 main religions in Barbarian Invasion: Catholicism (Berbers and Western Roman Empire), Orthodox Christianity (Eastern Roman Empire), Zoroastrianism (Sassanids Empire) and Paganism (Huns, Vandals and Sarmatians). Some of the factions, particularly the Roman factions, have diverse lands filled with multiple religions. The Roman Western Empire has a unique ability that allows them to mass convert their populace to support either the new religion of Christianity or stay with Paganism. In fact governors and generals are religious and have opinions which will affect their mandate of a settlement. The religion of the Emperor, at the time will also have an effect on the Roman Empire’s people opinions. A pagan Emperor will cause unrest in Christian-dominant cities for example. To some extent the religion mechanic has replaced the culture mechanic.
Horse Archer/Cavalry Dominant Civilizations – In the original Rome: Total War, the Scythians were the only horse archer civilizations. In Barbarian Invasion, horse archers have become a powerful new force in the game world, with factions like the Huns using them to a devastating degree against the more ‘conventional’ Roman Armies.
A more fast paced campaign – Overall the campaign map on Barbarian Invasion has fewer settlements and this results in each battle becoming more influential on the overall outcome of the game. With fewer settlements and with unit upkeep costs much higher than in the original Rome: Total War, raising an army is much more difficult and expensive. Losing that army is even more costly because players are not as able to crank out massive armies as in Rome: Total War. A massive battle as a result has the ability to decide the entire course of a war.
General Loyalty – Roman Family members have the ability to switch sides to the Roman rebels if particularly disgruntled. Other rebels also have the ability to join the side of the player as the civil war in the Roman Empire continues.
Swimming – Troops can swim across rivers rather than attempting to take, for example, a very heavily defended bridge. They move slower while swimming and can be shot at by missile units. When troops are exhausted and swimming they can drown.
Non playable factions are playable in custom battles
Western Roman Empire (playable) – Starts off with a large, but poorly defended empire in decline and must face off against most of the powerful barbarians and is often betrayed by the Eastern Roman Empire. Western Roman unit roster can vary between 'tamed' barbarian troops and Roman troops. In some cases the barbarian troops are almost worthless as their Roman counterparts are more loyal, stronger and cheaper to purchase and maintain. In other cases like the Sarmatian Cavalry, they are good value for money and have interesting traits mixtures like heavy cavalry with the fast moving trait of light cavalry. Western Roman must also deal with two conflicting religions. The state religion of Christianity (strong in Italy) and Paganism (strong elsewhere in the empire). The player has the choice to try and convert the populations of the empire to either Christianity or Paganism or try to live with both. Either way the player must also deal with huge debts being created in Rome, where his field armies are recruited, financed by trade and surpluses in the rest of the more backward lands of the empire. 'Vanilla' Total war games have never been known to have very hard single player campaigns, but this is an exception. Western Rome in most cases can be lucky to hold onto much of the original empire having to fight on multiple fronts with the Celts and Saxon in Briton, Berbers in New Carthage in North Africa and up to 6 or 7 hordes migrating into Roman lands across France and Southern Europe. The Romans usually fight defensively and must wisely pick their battles from fortified positions like key bridges. Most barbarians take land outside of Italy where most of the wealth is generated putting pressure on Roman coffers and recruitment. That and Roman settlements rebels make this by far the hardest campaign in any vanilla Total war game.
Eastern Roman Empire (playable) – Starts off with a well defended position, most barbarians ignore it and move onto attacking the Western Roman Empire. Main enemy is actually the Sassanid Empire, which left unchecked, can become very powerful. Religions is also an issue with the Eastern Romans as they must convert their populations from Zoroastrianism and Paganism to Orthodox Christianity. Unlike their Western Roman counterparts the East usually does not suffer from massive financial hemorrhaging and has access to a powerful, diverse and conventional roster of units including the best archers in the game. The Eastern will also eventually try to unite the two Rome by force as they are also tasked with controlling Rome.
Western Roman Empire Rebels (non-playable) – When Western Roman cities rebel they join this faction. Periodically some rebels switch sides back to the Romans.
Eastern Roman Empire Rebels (non-playable) – Eastern Roman cities that rebel join this faction and work in much the same way as the Western Roman rebels.
Romano-British (non-playable) – formed when the Romans left Britannia and have many advantages like Roman-British Legionnaires and faction specific units like ‘Graal Knights’ (super heavy cavalry as well as powerful artillery).
Celts (non-playable) – have units ranging from chariots, crossbowmen and druids
Huns (playable) – Start off without any cities, as they move towards the Eastern Roman Empire and then to the Western Roman empire they displace many other barbarian factions on Roman border. They have some of the strongest cavalry in the game, weak infantry and next to no infrastructure capabilities apart from using what they capture.
Goths (playable) – Very similar to the Huns in play style and victory conditions, expect with much better heavy infantry capabilities.
Sarmatians (playable) – A very cavalry specific faction whose aims are to conquer Dacia, with many unique units ranging from strong cavalry archers, heavy infantry and ‘Virgin’ female archer and cavalry corps.
Roxolari (non-playable) – very similar to the Sarmatians
Vandals (playable) – Start off as a horde in flight from the Huns with victory conditions involving controlling much of the Western Roman Empire
Franks (playable) – A horde capable faction that starts off with a single settlement outside the Roman border, they have very capable infantry. They aim to conquer Gaul from the Romans.
Saxons (playable) – Start with Denmark and aim to take Britannia and North France. Limited cavalry, but decent heavy infantry that hold axes and units that have shield wall capabilities.
Alemanni (playable) – A good selection of balanced troop types and a decent position to strike out in multiple directions mean the Alemanni can be a strong faction, even if surrounded by many aggressors. They aim to control Northern Italy (including Rome) and central Germany.
Lombardi (non-playable) – Almost identical to the Alemanni
Burgundii (non-playable) – Also very similar to the Alemanni
Slavs (non-playable) – The Slavs share many units with other barbarian factions, with no real unique units. They are simply a horde designed to simulate massive migration of Slavic peoples across Europe like the Huns and other barbarian factions.
Sassanid elephants in action
Sassanid Empire (playable) – The only faction that can train elephants, the Sassanid start off controlling much or Arabia, Turkey, Iraq and Syria. They have some of the most powerful cavalry in the game such as the Clibinarii, which can switch from being very capable horse archers to powerful heavy armored cavalry. Their aim is to take over the Eastern Roman Empire.
Berbers (non-playable) – A wide range of units, though few in numbers ranging from camel riders, heavy axeman and spearmen.