Here's some feedback.
Playing Tarth, I'm now ~200 turns in the game, normal world, challenging AI. I had no metal til around 150 turns and sparse crystal due to strong monster lairs right by the Crystal Crag. That would've been a problem if I wasn't playing Tarth
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The early game was hard due to 3 Troll lairs in my part of the world. My first hero has 5-6 wounds and really suffered (now he has path of barbarian and 125 HP so things are better, but for a while he was close to useless), my dodging spearmen didn't really dodge anything but things greatly improved when I built archers. But even then, every battle against troll armies was a close thing, especially when they had Shamen.
I have 7 cities, cleared the Troll lairs (some battles were really epic) and killed all weaker monsters. Now I have to decide where and how to expand next, and an Ogre lair is close by, generating big armies which is awesome.
Now some observations and suggestions:
- Corpse Spider has race: Men
- Umberdroths are too weak. After all the trouble with Trolls, I killed 3 Umberdroths without breaking a sweat. I'd suggest you improve their stats considerably.
- Cave Bear lair doesn't seem to improve or generate additional bears. There are only 3 bears sitting there. Is this intended? By the way, battle against them was great and I nearly lost it due to maul and sick bear HP. 
- Dodge doesn't seem to make much difference. I haven't noticed units that I designed as great dodgers to behave better than units with 0 dodge.
- Heroes are too cheap. And very much so. Level 7 or 9s for 5 or 7 gildar. I don't like that. Now I have 6 quite powerful heroes and I bought them for almost nothing. Also, I think they're slightly too many of them around, especially since AI cannot kill them due to their powerful armies.
All in all, I'm enjoying the game. Archers are kings so far against monsters, but I still need meatshields in front of them and still usually lose at least one group due to fast monsters. I'm looking forward the war against AI and siege.