That might be it. It might be both, I also saw Balkan undertones in the kingdoms and invasions but that might be my background.
In any case, with the Holy Roman Empire model, the Slavic analogy is even more off. Poland would be one of the northern kingdoms---but Silesia was German-speaking until 70 years ago. And Bohemia was a Czech-speaking kingdom, but it was a part of the HRE and had a prince elector of the Empire. Not to say that it's not inspired by the military history of Poland, of course it is. But whereas I can see the conflicts and battles based on Poland-HRE conflict, Nilfgaard always seemed more foreign.