Banking: The Babylonians did it, they might've started it, appropriate eh. So did the Israelis during Roman occupation or there'd be no tables needing turned in the Temple. One of the
exterior (?) purposes of the Knights Templar was international money transfer, and the Vatican absorbed this after the first black friday 13th. Recognisably modern banking originated in 14th century Venice, Florence, etc.
Politics: Politics has been around since, well, cities at least.
Medicine: Been around since the beginning too, in some form or other. Aside from the obvious - mutations, magic & many-races - I've never felt this to be out of place either.
Witcherworld is circa 13th century, its break with "our history" happened 1500 years ago, and they are very different worlds now. Nevertheless I've seen nothing in the Witcher series - apart from the blatant eggs - that I didn't think could be explained away reasonably. Sure they have tangible magic, alien races, and didn't lose a few centuries due to philosophical stagnation... if "our Earth" were to go to war with them
now, I suspect we'd be scuppered.
That said, I don't think either Mr Sapkowski or the REDs set out to create a historical fiction series, and there is really zero requirement on their part to be historically accurate. It is the nature of the Witcher to attempt to discern a modicum of rationality when it blends fantasy & folklore for a modern audience.
TL / DR ; I really don't see the problem
