Looking really good even unfinished. To partly answer your original question, early medieval equivalent folk with mediocre travel would probably have had many borders as natural barriers. Would have warred and consolidated but for the impassable river, or no travel over the impassable ridges anyway, so different societies developed and stayed apart. If you have decent sea travel, that could generate some colonies and discontinuous ownership. Assuming some armed conflict and some medieval intrigues, as well as some characteristics from earth's late feudal/ Renaissance periods, might well be some senselessly random borders drawn as the *current* bound of ambitions and empires in flux. THAT parcel went with the princess Ysmareldia when she was married off - dowry, y'know. THOSE lands stayed with the Baron when he went all nutzo and took up firebreathing, while the (now) free cities over there felt endangered by the more rational but repressive king across the valley, so they broke from the Baron and formed their own league. Including RiverCity over there still surrounded by Barony, AND PortCity over There, even though Wideflung Empire claims the land around it. That kind of patchwork would easily be plausible.

You can hint at past conflict and shifting by the names themselves - olden names obviously not a linguistic match with the current array of neighboring names can stay for centuries. Or could be adapted into NewLocalLingo. Yadda-opolis could shift to Yazda o'Politz HERE, where across the way Foo-opolis and Bar-opolis remain so-named, while more 'modern' cities like Fleuveville and Jaquesville show the French-equivalent conquerors' stamp.

'Course a militantly-OUR-society mapper could just as easily translate Tsei-p'teng, Gortiz, O'louipo'leia, and Butterton into the local language. <shrug> And you could produce maps based on ANY of their viewpoints. Likewise your in-character cartographer could assume borders where none exist, or optimistically (or politically-correctly) assume his King's latest campaign would be completely successful.

Your period could encompass anything from the splintered statelets of early Germany and Italy, to massive unified empires; Autarchies to city-states to hereditary monarchies to republics to nationless expanses of tribes and clans. It's a rich palette; can't wait to see what you paint in!