This is going to be terribly amateurish and thirdhand, but...

I know in some of the DVD commentaries and behind-the-scenes stuff they've mentioned that Rohan's production design is basically mock-Viking. I also know that there are just a very few extant Old Norse stave churches. I've been in a replica of... I can't recall whether it's all or part of one of these churches, but believe it or not, it's at EPCOT center in FL in what I like to call Fake Norway.

Here's a page that discusses stave churches - no, they aren't just like Edoras at all, but you can see some common design elements. What sticks in my mind is the carved knotwork panels in dark wood, very similar to what you see both plain and gilded in the films. I don't really know how much other Viking architecture is left, so there's a good chance that extant stave churches were in some way a major source, even if they were just used to compile books on design that the filmmakers later referenced.

http://www.medsca.org/
and the EPCOT replica is here -
http://www.medsca.org/stav_rep_epcot.html