I've been working on a campaign in Asia and the Indian Ocean, starting about 780 AD, with an emphasis on maritime trade.
In medieval Europe ... I'd probably do North Germany (only because I've lived there), and I would again go for early-ish Middle ages. That's Charlemagne's era in nearby what is now called France, and includes the conquest of Saxony, but I would want to include land east of there out to Brandenburg or so, and maybe as far south as the Harz mountains, but that makes the area bigger than I'd want. The lower tech includes agriculture still on the two-field system, fewer kinds of hardware.