Okay, so I've just spent the past three-ish hours exploring the early-game single player. Here are some deets:
At its core, Ymir is about A.) building a strong agricultural base to support ever more developed economies. (more on this in a moment), and B.) ensuring your security through armies and defenses. After playing I stand by my statement that it is very Land of Lords-esque. Think LoL mixed with AoE and maybe a little Settlers thrown in there.
Economy and resource management seem to be at the forefront of the gameplay, with a focus on not only making self-sustaining cities, but later acquiring more land from which to support those centers of industry. After a somewhat slow hunter-gatherer start, you're off to the races once you research farming, which will allow you to start building up a standing army with which to explore, or at least a well equipped militia (which will ensure your safety for some time).
Speaking of research, I think that is, imo, the strangest aspect of the game... it appears to be a semi-random progression, with certain research options appearing based on what you lack (among other things). I.E. You struggle with gathering food, you will be able to research farming quicker. Running out of water, and wells become an option. etc. I don't think this is the only/deciding factor, but it does appear to play a significant part.
Combat is pretty simplistic. From a defending perspective you can have different armies (either actual standing armies or need-based militias) assigned to multiple defense zones and from what I gather use fortifications to keep your ranged units safe while they pick off the enemy (although I always seem to be losing one or two javelineers). As an attacker your options are basically go-right-at-them from what I can see--to be fair, I am still only in the early game. Combat is... bearable, I can see why people criticize the AI and I have to admit its probably the weakest part of the game rn.
EDIT: I should mention managing squads, garrisons, and force composition is pretty important too... you can get rebellions if you improperly maintain your forces.
There also seems to be a much larger strategic element about claiming land, exporting and importing goods, and grouping up that I have yet to scratch the surface of.
There is still a lot to be seen and done, so make of this info what you will, but I will give it a cautious recommendation. I think this game could be awesome.