Just take a look at the town sizes in the late dark-ages, early middle-ages in Europe, ESPECIALLY in britain. If you look at the city sizes in that period, they do not exceed several thousand people, simply because there was not enough infrastructure to sustain them. The society at the time was a mostly self-sufficient population with minor local trade and maybe SOME naval trading.

Although several hundred years do separate the two eras, this principle would have been true for the barbarians too. Until the late middle-ages period, the barbarians were greatly technologically backward compared to the Islamic countries (which would be your "civilised" factions in RTW, like Rome, Greece etc).