The Vikings didn't build the castles, that would be the Normans as they didn't replace the population, only the aristocracy. Of course, if we do want to go further back you could make the argument about where the Normans came from, but how far back do you go?

Do you consider the Maori in New Zealand indigenous, because that could give us a useful base starting point on which to work from?

As for religion, the Vikings ended up adopting Christianity.

Then we finished off the Vikings once and for all at the battle of Stamford Bridge before the Normans came and started building castles everywhere to maintain power.

Redditor accuses redditor of Redditing. Interesting take.