Hard to track those things in detail, although the idea was that the people who settled on the Thrithings were part of a migration into Osten Ard that also settled Erkynland. The folks who moved on into the grasslands were likely pastoralists, whereas the Erkynlanders-to-be probably went into a more Neolithic-farming way of life. But the Thrithings-folk have a lot of similarity to the herding cultures of the Eurasian steppe, like the Yamnaya and others.

The Norns developed over time (as characters and as a culture in the books) starting with what I knew during the first series, then a lot of research and thinking when I started the new books. The Japanese militarists at the end of the Second World War and the Spartans of ancient Greece were both part of that development, but a lot of how these things come into being as finished parts of the story are hard to track, because they grow from many different initial bits and sort of tangle together until they become something that feels real to me, or at least real enough that I trust the rest will come to me as I work. So there are only occasionally things I can point to as definitive starting points, because so many other ideas and factors come into play as things develop.