Rome: Total War: Barbarian Invasion is the first expansion to Rome: Total War. It takes the setting of Rome: Total War forward by four centuries to the closing years of the Western Roman Empire, and features a wide variety of new features on top of the existing features of vanilla Rome. It is something of a spiritual predecessor to the later Total War: Attila, featuring many of the same concepts and settings as Attila, and many of the concepts set forth in Barbarian Invasion would reappear in Medieval II: Total War and beyond.
Barbarian Invasion was bundled in with Total War: Rome Remastered featuring multiple new enhancements and features. Alongside all the features added in the general remaster (improved unit models, graphical updates, etc) came new mechanics and new factions. Merchants were added from Medieval II to both the base Rome and Barbarian Invasion, bringing with them a new problem facing the Western Roman Empire, being debased currency. West Rome is unable to recruit merchants until their debased currency has been fixed. The remaster also made ALL of the factions playable, such as the Celts, Romano-British, Berbers and Slavs, giving players more of a choice for playing as the barbarians and taking down either one or both of the Roman Empires!
The Hunnic faction is distinct from the Nomadic factions, for they are only represented by the fearsome Huns themselves.
- Huns: Playable. Their army is primarily made up of large units of cavalry with high endurance. They start as a horde near the Roxolani and Alan tribes north of the Black Sea. The Huns have some of the best horses in the game and have very skilled horse archers, however, their infantry selection is limited. They also have a lack of infrastructure and must often rely on captured buildings such as ports.
These barbarian factions start in Eastern Europe. "Nomadic" is an umbrella term; it includes both Iranic tribes and eastern Germanic tribes, as well as the Slavs. The Nomad cavalry is overall superior to that of their Western brethren; furthermore they have access to highly competent mounted archers. Their infantry should not be discounted, as it is also often of good quality. The main drawback of the Nomads is their lack in siege technology. All Nomadic factions are able to form a horde.
- Goths: Playable. Usually described as sacrificing captured enemies to their pagan god Tyz, in the beginning of the game most of them are already converted to Christianity. Their strength lies in their cavalry, although their advanced infantry is of high quality as well. They begin in the area of modern Romania, although they are soon displaced by steppe nomads.
- Vandals: Playable. Settled in Rome's former northwest African provinces. Start as a horde located northwest of the Black Sea. Their armies emphasise horsemen with infantry in support. They are capable of adopting Christianity and are able to train mounted priests.
- Sarmatians: Playable. Tribe living northwest of the Black Sea corresponding to modern Ukraine; they have high quality cavalry and woman-warriors, yet a very limited selection of infantry. Luckily, the main infantry unit of the Sarmatians, Bosphoran Infantry, are effective warriors against infantry and cavalry; this flexibility reduces the need for a larger unit roster.
- Ostrogoths: Non-playable. Similar to the Roman rebels, the Ostrogoths are spawned when a Gothic city revolts. They have the same units as the Goths.
- Roxolani: Non-playable. A tribe similar to the Sarmatians (they have the same unit roster); because they are neighbours to the Hunnic hordes north of the Black Sea, they tend be the first faction to fall to the Huns. Afterwards they form a horde as well.
- Slavs: Non-playable. The Slavs appear later in the game around 410 A.D. just east of the Baltic Sea. They have a similar unit roster to that of the Vandals.
Barbarian factions populate western and central Europe (at least those parts that are not under Roman rule). Their strength lies in their powerful melee infantry, and most of them can lay sieges like civilized factions. The Barbarians do mostly roam more or less mountainous terrain with many forests; hence, their cavalry is a bit lacking and artillery is rare.
- Saxons: Playable. Start with only one settlement in the Jutland peninsula. They have access to top-notch infantry, all tiers of ports and are a pure pagan faction (most other factions are a mixture) making internal stability easier. They are, however, limited in cavalry, artillery and tactical infantry. They are pagan and worships many gods; the player can build temples to three of them; Woden (Oden), Ull and Donar (Thor).
- Franks: Playable. Eventually settling in Rome's former Gallic provinces, the Franks begin in what is now central Germany. They have axe-throwers and durable infantry, and can form hordes. They are also one of the few barbarian factions with decent cavalry, in the form of paladins. They can build temples to the gods Woden (Odin) and Donar (Thor). They are capable of adopting Christianity as well, and are one of the few barbarian factions able to build onagers and ballistae. If they adopt Christianity, they can train Catholic Priests and Paladins. The Franks are the most technologically advanced Barbarians, as they are able to build paved roads and have farming techniques on a par with the Romans.
- Alemanni: Playable. A confederation of Germanic tribes, their powerful infantry includes fearsome berserkers. They begin in the area of modern Switzerland, and are tasked with expanding around that area.
- Celts: Non-playable. This factions begins in modern Ireland and Scotland, and will be a threat to either the Western Romans or the Romano-British. Their army is composed of old-fashioned units, such as chariots and gallowglasses (infantry armed with large, two-handed swords), and berserker-like "Hounds of Culann".
- Burgundii: Non-playable. They tend to be rivals to the Franks, as their goal is to settle in Gaul as well. Their army contains powerful berserkers and other warriors, and they too can form horde groups. They have the same units as the Alemanni.
- Lombardi: Non-playable. A strong all-around nation, they seek to invade Italy. Their army is primarily infantry, including berserkers. They can also form into a horde, but are otherwise the same as the Alemanni.
- Sassanids: Playable. The new ruling dynasty of Persia; the strongest power in the East, and essentially the offspring-kingdom of the previous Parthian dynasty of Persia. They are rivals of the Eastern Romans, and are the only faction to follow Zoroastrianism, as well as to still use war elephants. Their signature units are the excellent Clibinarii Immortals which accompany their generals. The Sassanids have (in comparison to the former Eastern factions) very strong infantry called Sughdian Warriors which equal the Roman Comitatenses.
Northern African ("Carthaginian") faction
- Berbers: Non-playable. Found in the area of modern Algeria and Morocco, they concentrate on desert warfare, relying mainly on lightly armored, fast-moving units; they have a fairly restricted unit roster, and are lacking in technology (especially infrastructure). They are a Christian only faction.
Roman factions include the Western and Eastern Roman empires, the Romano-British emerging faction and the Rebels of the Roman Empires (Eastern Roman Rebels and Western Roman Rebels). The Roman culture, together with the Eastern one, have the most superior technology in the game.
- Western Roman Empire: Playable. The Western Roman Empire is the most challenging faction in the game. There are several reasons for this: first, because the economy is poor and underdeveloped, second, because there is enormous potential for unrest and riots, and third, because it has many strong barbarian factions at its borders, preying on its provinces. The Romans have many strong units, like Comitatenses, Foederati Cavalry and Infantry and siege tools like Heavy Onagers.
- Eastern Roman Empire: Playable. The situation in the eastern part of the empire is much better than in its western side. The economy is rich and abundant, and the army is mostly strong and stable. There are, however, two little problems: invasions of Barbarian tribes that can hurt the economy when one of the major cities is exterminated by one of these tribes, and of course, the Sassanid dynasty in Persia that has been threatening the peaceful empire for years. Religion may also cause great problems. Its strong units are Equites Catafractarii, Equites Clibinarii, Legio Lanciarii, Comitatenses and more. They have the same siege tools as the Western Empire.
- Western Roman Rebels: Non-playable. The Western Roman rebels appear when Western Roman cities rebel, similar to the Eastern Empire rebels; their units are the same as those of the Western Roman Empire.
- Eastern Roman Rebels: Non-playable. Like the West rebels, they emerge only after revolt, in a city of the Eastern Empire. Same units as the Eastern Roman Empire.
- Romano-British: Non-playable. The Romano-British are the only emergent faction which only believe in the Christian faith, and the second in the game together with the Berbers. The faction emerges when one of the barbarian tribes sacks a Roman city in Britain. The Romano-British have powerful units, including the Graal Knights.
The Rebels are present in Barbarian Invasion as well, but this time they do not hold so many provinces at the beginning as in the original game. However, every time a city is sacked by a horde, it becomes rebellious (since the horde faction will not settle down).
Like the base game, Barbarian Invasion also features historical battles mimicking the real life famous battles of the time period. However, unlike the base game, Barbarian Invasion only features two battles.
- Battle of Badon Hill: This is a semi-historical battle that occurred between the mythical King Arthur of the Romano-British as he fought off a large Saxon army at some point between 490 and 517 AD.
- Battle of Chalons: This is a famous battle that was one of the last ever large scale battles fought by the Western Roman Empire. The battle took place in Gaul between Atilla the Hun and a combined army of the Western Roman Empire and Goths in 451 AD. The historical result was a loss for Atilla and the end of the Hunnic invasion of Gaul.
- The two Roman Rebel factions were the only factions to not be made playable in the remaster of Barbarian Invasion.