Also, before any misconceptions are made:

The Barbarians aren't "Barbarians" as we know them. They are Barbarians as the Empire knows them. AKA: Anyone not under the fair rule and advanced political system of the Empire that gave everyone freedom and whatnot. They're not from the Empire, and hence they are lesser beings, especially because they have those weird opressive forms of governments! Pff!

This means that the Barbarians are actually Feudal Kingdoms, much like European Kingdoms of the 13th-14th century, attacking us because they're being driven to us by a larger tribe of real 'Barbarians', or the Huns of our IC world that drove the Barbarians into Rome. Probably a people riding horses a lot. As a result, the barbarians we will face are actually pretty much identical to us except that their armies are equipped less well, and are not at all centralised (aka, we get equipment from our military, they have to buy it themselves). They are most likely peasant levies and men-at-arms etc.