A belated post, but you must read Sarah Bond's fantastic piece for Forbes on barbarian soldiers in the Roman Empire and Game of Thrones:
Just as Jon Snow promised land to the Wildlings, many Goths were settled in arable areas of Thrace (modern Bulgaria) and then drafted into the Roman army in order to solve Rome’s worsening recruiting problems. As the Roman historian Ammianus Marcellinus notes, the Roman emperor Valens was initially praised for bringing in such a flood of recruits “from the ends of the earth” that could take the pressure to supply troops to the military levy off of other Romans in the provinces. Sure, they had allowed barbarians into the gates, but these barbarians could take the onus of military service off of them!
Read the full story here. It is also worth noting that her new book Trade and Taboo Disreputable Professions in the Roman Mediterranean, is out soon. Get your library to order a copy.