Something similar to Northgard

Entirely different gameplay (more turn based strategic combat, less city building), but on the mythology front The Banner Saga and Expeditions:Viking might be right up your alley.

Valhalla Hills - Norse theme + base building

• It's a bit simple I guess, but I've enjoyed this game a lot :)

Edit: Was about to suggest Banished but Incariuz's beaten me to it :P

A few more that might peak your interest despite not having a Norse theme:

1. Islanders - Fits the latter part of what you're looking for >Building cities, peaceful music and slow-paced gameplay.
   Here's a video of the gameplay if you're interested.
2. Kingdoms and Castles - Had some pretty fun times here too. No video though :P
3. Rimworld - Anyone who has played the game before will have something good to say about it, or so I was told :)

Cultures maybe? Although personally I've only played Cultures 2 so I cannot really vouch for these but afaik the gameplay is very close. And 1 + 2 seem to be missing from steam but if it's not a must have for you they can be bought on GOG

More European, but... Banished is a good game, and has some decent mods. Stuff like Colonial Charters mod adds a lot more to the game, so once you've experienced it fully vanilla, you can get more gameplay out if it.

https://store.steampowered.com/app/242920/Banished/

The Settler series (the earlier ones). Probably better too.

Of course Age of Empires 2 & 3, and Age of Mythology due to RTS.
And Endless Legend if you liked the setting and the gameplay is kinda like Northgard, but it's 4X, so it may be a lot of time for you just one match and it's far more extense in features.

If you're not limiting yourself to something that's new and on Steam, I'd recommend The Nations.

It's neither Norse or Slavic, but there are three different cartoony alien races, each can produce two unique resources (other 4 you'll have to trade) and some unique buildings. The game has inderect unit control, meaning you can't directly control you units (apart from military ones), but you can mark, say, a tree to be cut. There are more than 20 resources, more than 30 buildings, about 20 professions.

There are three technological ages. To advance you have research a certain amount of technologies, for which you first have to build a laboratory and hire a scientist. With each technology level you can research better buildings (and have more buildings in total), but at the same time your people start to demand more. Also starting from the second age you have to praise the gods or they'll do something nasty to you, like sending an earthquake or a plague (on the other hand if they're are pleased, you may make a wish for something nice).

Your people have a happiness meter - if they have nowhere to live, have no job, there was not enough food and they couldn't have lunch (and later stages they also want access to some luxury items) - they'll be homeless jobless sad potatoes who may eventually turn into criminals (who if I recall correctly may even kill your other citizens). Then you'll have to build a police office and train an officer to catch them. Later you can re-integrate them back into the society by giving them a job and a house.

The game is mainly a city building / economic management. As for the pace, I'd say it's rather slow (not in a bad way) with the ability to pause and also change game speed (x2 and x4).

*its story mode

I have not played Northguard, but I've watched a little bit of it, so take these recommendations with a grain of salt.
Settlers 2, original or remake (they're both good, but the remake lacks Vikings). The civilizations here are cosmetic though, and it's not really based on Norse mythology. The gameplay itself would likely fit though.
Personally I think Settlers 2 is the highlight of the series. The first one is a bit too simplistic, and the later ones have less personality.

Going a little bit further from what you asked for, Majesty: The Fantasy Kingdom sim. It's a game where you build a fantasy town, hire heroes and send them out to kill monsters. Like Dungeon Keeper, you don't have direct control over the heroes, instead they wander around and do their own thing, based on their personality, and you need to set bounties on things to get your heroes to go where you want, and different heroes will react differently to bounties. Rogues, being selfish & greedy loves bounties, but they're also cowards who will run at the first sight of trouble, and will likely just try and take things when the're not out on a bounty, Paladins will wander around and slay monsters and protect the weak, but are less easily persuaded with gold, Wizards like to read in the library, Rangers like to explore the countryside and so on. Majesty 2 kind of lost this aspect, with the heroes not having nearly as much in the way of personality, and thus I think it's a far inferior game, sadly.
Again, no Norse or Slavik mythology here.

And stepping even further away from what you asked for, in terms of gameplay, the Konung series. Not a series I have much personal experience with, but it fits the setting you asked for, as it's based on Norse & Slavic folklore & culture.

Norse and Slavic mythologies
Building cities, peaceful music and slow-paced gameplay.

Crusader Kings 2, even though it's completely not what you expect :3.

RTS-like, Warcraft 3 is the closest what comes to my mind if we look at the goals and the game oriented on less but more powerful units, although it doesn't have a lot in common, and is fantasy on its own. Settlers for city building aspect. Rimworld for both, but once again completely not what you expect.

Northgard is kind of unique in how well it mixes up settlers and warcraft 3 mechanisms.

Let me make a list:
The Banner Saga
Expeditions: Viking
Valhalla Hills
Islanders
Kingdoms and Castles
Rimworld
Cultures
Banished
The Settler
The Nations
Age of Empires
Age of Mythology
Endless Legend
Majesty: The Fantasy Kingdom
Konung

played

Thea
Slavic mythology heavy, but you limit yourself with one village and no way to capture other areas.
Card battles are great. It's turn-based so it's much slower than Northgard in that aspect.
My review of Thea.

Posting a second time here - just spotted this on Steam and I immediately thought of your post since it's a Vikings-themed strategy game, I don't think the game has any city-building though!

Bad North