So how does it compare with the HoMM games?
There's some diplomacy: you can make trade agreements, declare war or peace, exchange recourses, maps and your children. (sons as captives, daughters as brides)
There's a quest system, you complete various quests and get rewards in form of artifacts, units or resources. Actually, very similar to quests in some recent strategy games.
Every faction has it's own victory condition.
Strategic map is more civ-like, units move individually (not in armies led by heroes). Units are individual, not stacked as in homm.
You can't build cities, but can build roads and castles. Road reduce movement cost, castles provide defence for nearby cities. Cities can be captured and absorbed or razed and later colonised. Cities generate income and food (needed for pop growth), but it's all very simplistic.
Recruiting system is interesting - it's somewhat similar to homm: you have a roster of units you can recruit in your capital, but as opposed to homm units don't appear every week, but appear when you do heroic deeds. (Razing villages inspires the viking youth to join the army)
Combat is even simpler: square grid, units either move 1 square or attack/shoot/cast magic, no initiative system. Meleeing units always kill each other, when two units start fighting, they exchange blows until one of them dies, ranged units shoot once per turn.
Units have only three stats for combat: melee, ranged, defence. Also movement speed, but it's for strategic map.
The most interesting thing is the concept of raids. There are different structure types in the game - cities, castles, monasteries, etc, but only castles provide good defence. (And they are used to defend cities and monasteries) So, most of the time the castles have walls and powerful troops in the garrison, while cities don't. When you attack a city and kill the defenders, you have 4 options: raid, raze, plunder or subjugate. If you raze, you kill everyone inside and get all the loot, subjugate leaves some citizens alive(more pop when you colonise) but you lose all your movement points and the next turn the castle's garrison would attack you. If you raid or plunder, you get less loot and kill only a handful of citizens, but you barely lose movement points and can escape the battle with castle's garrison. So you raid cities and immediately retreat (on a ship preferably). Good way to boost your economy without risking your troops. 100% realistic viking simulator.
It's also very pretty. Strategic map and combat background are beatuiful. Strategic map also has a nice detail - unexplored territory is not fog of war, it's an 'old map'. There are approximate locations of big cities, castles, rivers, and mountains, so you can get the rough idea where to expand without knowing what really awaits you.
So it's not homm, not civ\mom. Very unique game.