How do you research spells and what do you need to do it? I was able to open my spell book and when I highlight spells I see a "learn" button but when I click it, all that seems to happen is that it goes to the top of the spell book and says "learning". Yet nothing seems to actually be happening as the days keep on rolling by. Do I need a special building or Tech to do research? Does it cost anything to research a spell? How long does it take? It's frustrating trying to figure this out in game (it took me awhile to learn how to train units too since it was not very intuitive). I did notice however that I suddenly had access to Fireball, which is NOT one of the spells I was trying to research so I have NO IDEA where it came from or how I learned it. Any explanations on that would be appreciated as well. I would LOVE a real tutorial on this stuff so I don't feel like I'm constantly blundering around in the dark, but what are you gonna do?
Learning spells is like researching a tech. There are world resources and improvements that generate arcane knowledge that progress your spell learning. Unfortunately, these are just not in the campaign so you can't learn spells. It works fine in sandbox. It doesn't cost anything other than time. All the spells you get are from the story, if you read the text and watch your Campaign journal (click the Kingdom Report button on the very left of the bottom UI bar, then the Campaign tab) you'll get notices of when new things unlock for you.
On the subject of spells, what does Essence do? I wanted to cast Imbue Champion to see what it did but it told me that it was special and required Essence and wanted to be sure I wanted to do it. I ended up not casting it because I have no idea what Essence is used for, if/how it is replenished, or how valuable of a resource it is.
Essence determines how much mana you have and how many permanent enchantments/summons you can maintain. 1 point of Essence is 1 point of mana. Thus, the process of Imbuing a Champion allows them to begin casting spells (you give up 4 essence and they gain 4), and on subsequent level-ups they can improve their essence score for more mana.
Also, can you capture enemy cities? I'm playing the first part of the campaign and I killed the little goblin dudes in their city. Can I capture their city now? If so, how? I couldn't figure it out if it is indeed possible. Does it require pioneers? If so, please explain how to build them because they don't show up in my available units to train. By the way, shouldn't a battle taking place in a city have a battlefield that, you know, looks like a city? I was surprised when I started the battle and it looked like the same old grassy plain. Do units garrisoned in a city like that gain bonuses to defense or something? Wasn't sure because, you know, nothing is explained anywhere.
Enemy cities are capturable, but the thing you destroyed isn't a "city" as such. It's just a quest/event location. To capture a city, all you have to do is attack it and defeat its defenders. When you do attack actual cities, the tactical battle maps do reflect this (there are buildings and such), but I haven't gotten far enough in my sandbox to see what effect big walls have on tactical maps. Units garrisoned in a city gain an HP bonus.
I know the game is a bit light on easing new players in, but keep posting questions and some of us will do our best to help you out 
