Don't undersell the role that large urban centers play in the feel of an archetypal D&D setting. For me, lamplit dens of wine-sodden thieves, harbors flooded with the strange speech and goods of distant lands, and gorgeous palaces teeming with splendor and intrigue are all iconic vistas no less than villages, ruins, and keeps on the marches.

As with most settings, I'd be most interested to see you flesh out a single local region which serves as a good sample of the setting's core ideas, and also as a good adventure locale. Such a region, for me, ideally includes:

  • A starting Village, large but not big enough to be fortified, which is fairly centrally located and receives the most detail. Should have a sizable handful of named NPCs.
  • In your case, with medieval political structure being a stated goal, a fortified keep, some days distant from the Village, where resides the chief local aristocrat.
  • A smattering (five to ten) of smaller settlements, broadly sketched. These can be small farming hamlets, isolated homesteads, hunting camps, the manor homes of lesser aristocrats, or fortified trading posts. Each should have one or two named NPCs, and a one-sentence description of what makes it unique.
  • At least one settlement of a non-human but allied people; Elvish forest enclaves, Dwarf trading posts, Gnome burrows, Halfling villages. Politically independent but isolated within the land.
  • Wilderness regions: swamps, forests, hills, deserts, or coastal islands. These serve as barriers to travel via difficult terrain and their general inhospitable nature. They should also be the location of:
  • 3-4 adventure locations. Rumors of these places should reach the central Village. Should involve at least one ancient ruin, one natural environ populated by more 'wild' monsters, and one dwelling of humanoid-esque monsters.
  • A large metropolitan center, not present in the region itself, but in whose broader political and economic orbit the region exists. Can be sketched out in broad terms, and should be quite remote.