I turn on the grid an check out those 3 hills. I count tiles and, no we cannot settle there IMO.
I check the military advisor: 10 warriors. I will disband a few regular warriors.
I visit the military advisor, we have an average military compare to the mongols.
Too bad we didn't get some military a little faster
I check city builds.
Worker in Odense changed to barracks.
I leave the rest of my decisions to the IBT when i have construction.
Turn1:
Construction done, i change research to currency.
About 16 turns with a balanced cashflow, 13 turns when spending our gold on deficit with the current situation.
Since it is a decision that is easilly reversable if i later decide to do different (by running a deficit), i turn research to 0.
Bergen, Trontheim, Birka and Reykjavik are changed to aquaduct.
Disband warrior in Reykjavik.
Disband warrior in Bodo.
Start moving 3 veteran warriors towards city with barracks. I will upgrade them.
Turn2:
nothing.
Turn4:
Reykjavik: Aquaduct -> Barracks.
Turn5:
Aarhus: Horse -> Aquaduct.
I sell math to Spain for 52 gold and to america for 25 gold.
I turn on research again since else we will have too much money. 13 turns with balanced cashflow.
Turn7:
Stavanger: Horse -> horse.
Alesund: Worker -> Wealth for a while since this city will not be worth any city improvements and at this moment we do not want to pay upkeep for more workers.
Bodo: Horse -> Horse. (Subject to change depending on the corruption level after FP. At the moment this city's corruption makes investment in growth less valuable)
Sell math for 29g to the celts.
Turn8:
Trontheim: Aquaduct -> Horse.
Reykjavik: Aquaduct -> Horse.
Karasjok: Worker -> Settler.
Our money deficit is rising. But soon, we will be conquering cities.
Turn9:
Stockholm: Horse -> Horse.
Turn10:
Copenhagen: Aquaduct -> Horse.
At this point, we have a sufficient army to invade the Mongols.
We have Copenhagen, Stockholm, Bodo, Stavanger and soon Odense not having anything better to produce than horses. This adds up for a total of about 2 horses per 3 turns.
Therefore, i think we can swich Trontheim and Reykjavik to Libraries. But since i am the evil warmongerer and you guys like to build Libraries, i will leave that pleasure to the next player

Birka and Bergen must IMO get a Lib after the aquaduct as well. (See i'm not that evil a warmongerer)
We can also start producing Settlers again to fill the space we are about to conquer. I suggest to build some in our most food rich cities when they reach sizes where happiness becomes a problem. (Like Oslo) We can thus keep those above size 6 for military support reasons. The completely corrupt cities Karasjok and Alesund can also build settlers
Before starting the war, we should consider gifting the Mongols republic. If you gift them republic on your turn 0, i assume they will revolt right away. This will prevent them from pop rushing (which wastes our future population and happiness)
If you look at the economic picture, you will see that unit cost is indeed killing us. This can however be solved soon as we are gonna conquer cities, ICS that ground and have multiple cities about to grow past 6.