I guess I don't typically associate 2D sidescrollers with base-building, though when I think about it for a minute games like Terraria and Starbound do come to mind. But if I'm gonna compare Sons of Valhalla to something, it'll be a game more like Kingdom. It's a singleplayer action game that's got tons of vicious Viking combat, base-building, strategy, and even RPG elements...
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