"You say that all men who have fallen in battle from the beginning of the world are now with Odin in Valhalla. With what does he feed them? I should think the crowd there is large." -Gangleri
The significance of Valhalla in Norse Lore is undeniable. It is the Viking warriors heaven and belongs in any world in which Vikings reside.
I have not found many post here that describe what the Eyvindians believe, but I think that having Valhalla as a major origin point is crucial. There is already such a strong history that it would be easy to convert much of it to the Kiwike Realm.
Valhalla must be extremely hard to reach for the living. Taking months of travel through dangerous terrain and the unforgiving wilds. This could be a great way to RP the founding of the city. It should be one of the first that is established. Several of the Eyvindians set out to search for the mythical city. The distance traveled should be far, both securing a safe base of operations and strengthen your lore.
Since Valhalla is were Vikings go in the afterlife, I would honestly make all Vikings set it as home. If they die they would spawn there and have to make the journey back to the land of the living.
http://en.wikipedia.org/wiki/Death_in_Norse_paganism
Much has been written about the grander of Valhalla.
http://en.wikipedia.org/wiki/Valhalla
I have been roaming far and wide on the Temp map, thinking of things to strengthen future RP. I found a very tall mountain climbed it into the clouds and built a small hut. The clouds rolled through me and I thought this would be perfect place for Valhalla, in the Heavens.
Having the entrance in the clouds, the holy doors of the ancient gate Valgrind, tucked within a mountain. Going through the gates, descending down into a vast open city. It should be an amazing site to see. I look forward to seeing it one day.
This is just an suggestion with the RP side. Hope it helps.