The Dark Age world is a distorted version of our own historical dark age Britain and Europe. The game is set in 610 AD but things are not quite as they were in the real historical world. Major differences include:
More towns & cities survived the departure of Rome, so there are many urban areas.
Magic is much closer to the surface.
The power balance between celts, picts & Saxons is much more balanced, no single power has a significant ascendancy.
There is growing pressure from the continent, the Byzantine empire is not content to maintain itself but is beginning to develop an expansionist bent.
The Romano-British do not rule, but their great houses occupy positions of influence within British society.
Atlantis & Lemuria/Mu were real places. Atlantis and Mu were both magically powerful cultures who fought a great war. The magical remnants of this clash, tens of thousands of years ago, still reverberate through the known world.
Mechanisms are far more advanced than the real-world of 610AD, much roman and greek technology has been maintained and developed, partly with the assistance of lemurian and atlantean lore.
This game has nothing to do with any White Wolf game/setting.