In AoM, the norses don't have any archers. They do have axethrowers, but those are infantry units. In AoK, franks axethrowers didn't add any firepower to towers and castles. What will be the situation with the norses axethrowers in AoM??? Considering that they are weak on defense, to be unable to garrison units with more firepower than the vils is a drawback for them.
I know, they are an offensive culture, but they will need some defense here and then...
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Recall : in AoK, the added firepower to a building was not calculated by the numbers of units inside but of the total attack of those units. For instance, if they combined for the equivalent of 2 building-arrows, then 2 arrows were added to the original firepower. It could take 2-3 archers for this but 4-5 villagers for the same two arrows. We already know that the norses building are defensively weaker (less hit points), they might be offensively weaker as well...