Barbarian Invasion is an expansion for the PC game Rome Total War which takes the action forward to the period of the decline of the Western Roman Empire. The campaign begins in 363 A.D. and ends in 476 A.D. If the player has not achieved its campaign goals in 476, he/she can either choose to quit or to play on and dominate the campaign map entirely. The expansion includes simulation of the religious tension of the period, as three major religions (Christianity, Zoroastrianism and Paganism) fought for dominance. Also depicted are the great migrations of the Germanic and steppe peoples (notably the Huns) throughout Europe.

As Barbarian Invasion takes place 350 years after the end of the original game, factions and provinces are very different. The Senate still exists but no longer has any power (thus playing no role in the expansion), and Rome has become an empire, grown and then split into a Western and an Eastern empire, ruled from Rome and Constantinople respectively; Egypt, Greece, Macedon, Pontus, Carthage, Gaul, Iberia, most of Britannia and Thrace have been absorbed into the Roman Empire. The Seleucid empire has collapsed and has been absorbed by Rome and Parthia; the Parthian dynasty itself has been overthrown and replaced with the Sassanid dynasty. Dacia has been conquered by the Goths, and Numidia has been entirely replaced by Berbers. The Huns have invaded Europe, and several major Germanic tribes have unified and strengthened to the point where they are now capable of invading the Empire itself.

Hordes are a new feature which is introduced in Barbarian Invasion and is a privilege to some barbarian factions. When such a faction loses its last province (some factions, such as the Huns and Vandals, begin with no provinces), rather than being destroyed it becomes a horde and is forced to settle elsewhere. Hordes are very large armies, representing an entire nation, however, they do not require upkeep. When a horde army successfully seizes a city, they are given the option of sacking the city (which does massive damage to the population and buildings of a city, giving the horde a large amount of money) or of settling in the city- which allows the horde to begin anew, with the new city as its capital; the faction then is able to conquer other provinces normally. When the horde settles, a portion of their horde army is disbanded and the population distributed into the city.

Horde factions do not "die out" unless defeated on the battlefield or all characters die; when they have finally found a new homeland and it is successfully conquered by an enemy, the whole faction becomes a horde again and flees, in order to seek a new home. With the concept of a horde the developers tried to reflect the migrations of several "barbarian" peoples of that time - especially Huns, Goths, Vandals, Franks, Burgundians and Lombards.

Victory conditions vary, depending on which faction is played. Each playable faction has it own territory requirement, including specific named territories--adding a layer of strategic complexity. Each faction is listed with a difficulty level relative to the other factions (apart from the game settings difficulty selected by the user). The Western Roman Empire may start the game with a lot of military and economic might--but everybody wants a piece of them, they require the most territories to win, and one of those required territories is the most contested (Northern Italy). The Saxons start small and do not have access to advanced technology, but they have a much smaller territory requirement--though they still have to contend with neighbors to the south who are crowding north to meet territory requirements.

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