The world of the Dyfed campaign is known (at least to Latin-speaking people) as Terra. It is the world you know… almost. Julius Caesar did indeed lead Roman legions to conquer Brython. On the other hand, Marcus Antonius did not fall in love with the Egyptian Queen Cleopatra - he did, however, get into trouble with one of the daughters of Pharaoh Ramses III.
Terra is part "what was" and part "what might have been". Some important historical cultures are adopted almost whole, others never existed. Real life and mythology walk hand-in-hand. Maps might look the same, but mountains are taller, seas are deeper, and the land is wider - a perfect place for Dungeons and Dragons characters to seek their fortunes.