"The campaign in Barbarian Invasion starts in AD 363, several centuries after the campaign in Rome: Total War. Rome has been divided into eastern and western empires, with the east ruled from Constantinople (now Istanbul) and the west still centered in Rome itself. Meanwhile, many of the old tribes seen in Rome: Total War have faded into history, and new tribes, such as the Vandals and the Goths, have emerged in the wilderness beyond Rome's borders. Your mission in Barbarian Invasion is to take control of one of the 10 major factions of the era and survive and expand your empire, mainly through conquest.
All factions are available to play immediately at the start of the game--you don't have to unlock them like you had to do in Total War. In addition to the aforementioned factions, you can also play as the Samartians, the Germanic Frankish tribes, the Saxons, the Sassanids, the Alemanni, and the dread Huns. Of course, each faction has its own specialties and bonuses. The Huns are superb cavalrymen, capable of shattering mass formations of infantry. The Franks, on the other hand, have superior warriors; the Saxons are excellent seamen, and so on. And, once again, some factions are easier to play than others, thanks to the benefit of starting position. Western Rome still controls a large swath of Europe, while the Franks find themselves hemmed in on all sides by hostile neighbors at the start.
As expected, the campaign itself is relatively unchanged in terms of the basic gameplay mechanics. You once again must conquer provinces by capturing cities. Then, you manage the growth of the province by constructing improvements, such as temples, barracks, stables, and more. You can then raise armies and go campaigning to conquer more provinces, or defend your territory from attacks by your enemies. If you played Rome: Total War, then you'll feel completely at home with Barbarian Invasion. There are some minor differences in Barbarian Invasions to note, though.
The tribal factions in Barbarian Invasion feel a lot more fleshed out than the tribes seen in Rome: Total War, and they have more building and unit options than their predecessors. We suspect this is largely because they're meant to be played on a near-equal footing with the Roman factions, but also because the barbarian tribes themselves were more sophisticated by this time. So in addition to building sacred groves to various pagan gods, you also begin to see Roman concepts such as sanitation creep into the barbarian's technology, allowing the barbarians to build larger cities.
Religion is a notable new addition to Barbarian Invasion. Rome's adoption of Christianity helped propel that religion to the forefront, and this is reflected in the game as well. You're continually notified which religion (ranging from paganism, Christianity, Zoroastrianism, and more) has the most adherents."
Rome Total War: Barbarian Invasion is scheduled for a 9/27/05 release.