Fortress Cities

I find them pretty useful. In addition to the level up bonus, a developed fortress gives your units +def, +accuracy, and (I think) +attack. Additionally, a well placed fortress can really make your defenses strong if you are confronting a faction you're not ready to eat due to additional defensive units plus walls. You don't necessarily need a bunch of them, but one or two developed fortresses are a pretty good investment.

In my plays, I wait until I find a tile with three available essences and then I plunk a fort down right on top. With Auras of Might, Grace, and Vitality (and Arcane Forge if you have a Scrying Pool) I can pop out some pretty powerful units, even late game.  If you're playing aggressively (or being forced into wars by the AI) I'd say have as many forts as you have "fronts" so each can replenish your stacks. For example, one fort was plenty in a medium map as I had one stack going from place to place (and I was attacked fairly early on). I didn't have a fort until mid-game when I finally found the area I was looking for.  I imagine larger maps two will do just fine.

You do get forts by taking them from the AI, but JonSarik is right that those forts typically do not have any essences.

I find fortresses very useful; though I tend not to make too many of them.  They really are the best place to maek troops by far though;  the bonuses to base armor and accuracy make a large difference, adn the cost reductions let them spam troops much better.  If they happen to get large they also get some nice bonuses at higher levels; and all the current level 3 city choices are quite good.

It's of course even better at a place with high essence to add enchantmetns; but even without that it's still some great bonuses for troops.

If you find one with plenty of materials, but not so many enchantments that can be great too. Helps you power through all the upgrade buildings, and start churning out stacks like crazy. Really useful if you don't have Arcane Forge or another suitable location. The enchantments when used with lots of essence can really turn a battle. But with 1/2 essence its really nothing some optimizing/streamlining and a bit of crystal can't add.

A lot of my units just come out of (Mancer) towns, with the best ones slowly coming out of fortresses (Knights/Mages). No reason you cant put one of the unit buff enchants onto a town to squeeze a little more out of a unit either. Unless you're tight on mana of-course.

Playing higher level difficulty AI tend to have ridiculous amounts of material/essence on their forts when you capture one. My favorite kind of fort.

Towns are the #1 priority for building (gildar income), but I find having 1-2 fortress can make your military better by being the place where your units are trained while the other towns provide the money/research. This work best when:

1. Your fortress has very high production. 

2. The fortress has 2-3 essence. This is for enchanted hammers, and Heart of Fire. Heart of Fire adds 1 fire attack per essence. While HoF is expensive at 100 mana, if you produce lots of units from that city, 2-3 extra fire damage per unit can really start to add up.

Archers, in particular, can really put that extra 3 fire damage to good use.   

3. You have armorer, barracks, forge, training yard, etc. Combine high productivity with these bonuses means you're churning out solid troops quickly, at reduced resource cost.

4. You want to build a lot of henchmen. Building them from a fortress starts them ahead a level, and gives them all those wonderful attack/defense bonuses as well.

(The more towns you have behind the fortresses, the more you can pay to rush production out of your fortresses, boosting their productivity further. )

I would guess the optimum ratio is something like 1 fortress for every 5 towns?