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.
Before we get to Vikings swinging axes and lobbing flaming pitch at each other with catapults, I have to point out how utterly beautiful this game is. There's some really tremendous pixel art and finely detailed animation, not to mention a day and night cycle and weather effects ranging from heavy thunderstorms to murky fog. I could just stand in the world and look at it all day if it weren't filled with enemies thirsty for my blood.
In Sons of Valhalla, your dad's been killed and your beloved has been abducted by your enemy, a jarl named Valgard, and you swear to rescue her and take revenge, even if that means hacking the limbs off everyone in England. You're not just a warrior, you're also a builder, and you'll construct, maintain, and upgrade structures like barracks, fishing huts, and mead tents to spawn new soldiers and provide weapons and sustenance.
Once you've got some loyal followers, you can lead them on a bloody quest toward the right side of the screen, where you'll battle enemy villagers and soldiers keen on stopping you. Raiding enemy towns will give you more resources you can use to make your own base stronger.
You're a nimble Viking, capable of dodge-rolling, light and heavy melee attacks, and ranged attacks with a bow, and as you progress through England you can upgrade your skills and combat abilities. Unfortunately, you're not the only one with a base, and both you and your enemies can build and use siege engines and flaming arrows to destroy barricades and gates and invade each other's strongholds.
Just don't forget to take the relatively few peaceful moments and just enjoy the scenery. Sons of Valhalla underwent a successful Kickstarter campaign earlier this year and projects a launch in December of 2020. You can find it here on Steam, and there's a lovely and violent trailer to gawk at above, too.