OK, so this is my two cents about the culture thing.
There isn't anything inherently wrong with borrowing from a Scandinavian/North European setting. It would look bad on us if we required all the cultures in this world to be carbon copies of specific eastern cultures that exist in the real world, since that sort of rigidity makes us look incapable of actually understanding how culture works at a very basic level. I read your app and it didn't feel like you directly ripped everything from the vikings. That is good.
However.
Your app does seem to suggest (if I read it right, and I might not have since I just woke up), that your people have been around a while. I take this from their supposed connection to the tunnel-building Visha. This does make their cultural uniqueness somewhat suspect. If they had been tied to a monolith such as Visha culture, and they have been in the region for the hundreds of years required, there should be some cultural diffusion. You should probably read all the other apps, particularly Vos and Daria, to get an idea of what life would be like for your neighbors, and be aware that the more general Visha culture would have been absorbed by your people in some way.
One possibility, I would think, is that they come from across the sea and raided up the rivers until they settled and made some sort of pact with the Visha. Maybe it was simple colonization where the Visha wanted people in that region who were strong and capable of governing it properly, or maybe the Visha employed them as a sort of Varangian guard. Either way, once they are there you would see their culture start to mix with the natives. They would still retain their boats (since they are practical), their Gods (though perhaps the added twang of Visha philosophical religion would cause mystery cults to become vogue amongst some of your people), and hints of their style would still be present mixed, just in amongst the native.
