I downloaded and installed a fun little freeware map-making program called Wilderness Mapper the other day, and decided for my first project that I would map out the old Duchy of Brittany, circa 1066 AD. At the time Brittany was a powerful independent polity which had defeated the Kings of France in several wars, and was strongly allied to the (also) independent Duke of Normandy - who would shortly take the English crown. It was still almost entirely Breton-speaking, and so should be more properly rendered the Dugelezh Vreizh. Its vassals included the Counts of Naoned (Nantes, in French), Gwengamp (Guingamp), Kernev (Cournouaille), Poher (Poucher), Roazhon (Rennes), Gwened (Vannes), Leon and Penteur (Penthievre).
The map isn't finished yet and it looks nothing like as good as the ones Alexis posted here. (Alexis is an ornery individual, but his world building entries are nothing short of inspirational.) Nevertheless it took me bloody ages last night to get even to the stage I'm at now. The basic framework is in place now, and the next task is to populate it with villages, hamlets, towns and adventuring locales. Each hex is 4.1 miles across; the dark green is deep forest, while the lighter green is just 'forest'. (In the early Middle Ages, Europe was still almost entirely forested.) You'll notice that the Eastern section is mostly untouched - that's because it's France, not Brittany.
Of course, the history of Brittany needs a bit of D&D-izing. My first impulse was to include elves in the mix somewhere (if elves are going to be based on a real-world culture, it's pretty much always going to be Celtic or Latin types). But elves bore me, and I think I've already written before about doing something different with demihuman races vis-a-vis real world cultures.
No, the people of the Dugelezh Vriezh and its environs are humans and dwarves, and I think it would be an interesting twist to have the dwarves be culturally 'French'. Indeed, the populations of modern day France, Northern Spain and Italy, Slovenia and Romania are all varieties of dwarf; they represent the gothic barbarian races who supplanted the celts in those areas and became latinised during the latter period of the Roman Empire.
Brittany, like Wales, and Galicia and the Basque country in Spain, is an area where humans remain dominant. The population is almost entirely monolingual in Breton, although there is some 'bleed' of dwarven French culture into the nobility, and some intermarriage resulting in a subculture of half-dwarves living on the outskirts of society.
Brittany is covered by thick forest and its population is almost entirely coastal. Inland, roving hunter-gatherer bands of gnolls can still sometimes be a threat, and monsters of various kinds inhabit the deepest and thickest glades. But the main dangers to the Bretons come from overseas - trollish viking raiders from Scandinavia and half-orc pirates who set sail from the southern coast of Wessex. (England is a melting pot mix of orcish Angles, Saxons and Jutes and human celts; interbreeding has resulted in a race which blurs from the fully human to the fully orc - with everything in between represented.) While relatively powerful, the Dugs of Vriezh find themselves threatened from on all sides - as the peace with the dwarven Kings of France is always uneasy.
I'll expand on some places of note in Brittany later.