(This was written before your post, Counlin. I'm letting it stand.)

That's the point. For any fantasy world I create, I set limits. In this case, "no non-human intelligent species" (that might be better adapted to cold) is one.

Without a reference frame, it's no fun for me. I want the familiar feel. This is, as I said, why I like geofiction, and it's why my principal (long-stalled) fiction project is pulpy science-fantasy. If I want to play with, say, Aztecs, my first choice isn't to make a world with an analogous culture; it's to work in alternate history or fantasy set on Earth, where actual Aztecs exist(ed).

Back to the "powerful Arctic civilization", this is related to another of my musings: how to make a truly anti-colonial world? That is, not just one seen from a perspective unsympathetic to the colonizers, but one where empire-building wasn't practically beneficial? The problem is that the real world isn't neutral to empire-building; it favors it. For a world to be even neutral to it, it has to have something that, relative to the real world, creates a bias against empires. For example, there must be negative economies of scale.

And both of these ideas came from something I heard about Dungeons & Dragons. That is, that people in its world are, on average, richer, better educated and more socially mobile that real medieval or ancient people. And I wondered "Can I make a world where that makes sense? Where, in a time before guns and steam, people can be wealthier than they were in reality?" And I realized the importance of the agricultural productivity limit. Without technology (or mass-produced magic used like technology), there are some things easy to imagine changing, but some harder. For example, I know of various fantasy ways to reduce disease and increase life expectancy (Purify Food and Drink wouln't be an unimportant spell in the ancient world...), but what I really wanted was a world that didn't have to be 90% farmers.