-one man army to breach formidable defenses

-until now (the first two levels), Vampiric Touch seems rather powerful especially with very fast attack speed (Firehand Gauntlets). Maybe you'd think it might suit better either as the normal Vampiric Aura or a spell based on Vampiric Potion.
-make the gnolls stronger; they barely deal any damage. Don't be afraid to create higher levelled neutral units like in the TfT campaign. They even drop a Rune of Healing

-again, too many items; you basically find them almost at every corner.
-maybe some puzzles or places to open the gates instead of just attacking them down?
-why would they keep the remains and even tell about them when the enemy is around? Maybe another way for Mal'Ganis to get the info through a Shade or something? Why were they even thinking a demon would specifically come for the remains when it was obvious he came to hunt them down as they ran to the island?
-the guards at the gate are horribly weak. That should not be for good defense's sake.
-a fountain of health on such a small map? Too easy. Did you forget about Vampiric Touch or you thought some might not use it from the first two levels?
-guard groups shouldn't be easily dragged one by one. They should all attack if one of them sees you. Maybe even make some of them trying to alert for backup. Say, spawned from invulnerable/doodad Barracks when the guard reaches it. Then, the alerted guards start to go after Mal'Ganis.
-I suggest researching the max upgrades for the enemy. Very fast attack speed+life steal+Orb of Corruption=close your eyes and relax.
-the Nobles should be miniboses, if not heroes, at least strong high level units. If some of them are supposed to not be fighters, at least their guards should be powerful. Every noble should be unique with its own set of abilities.
-Fountain of Health+items/runes of healing+/-Vampiric Touch. Imagine that... If you want a fountain for aesthetics, put a doodad.
-You don't even need the zombies. Mal'Ganis is far too strong.
-Would be nice to make the villagers run in their houses and to be able to get them turned, to bring their homes down first. It's plastic seeing them waiting and sitting idle there.
-maybe a zone where they try to ambush the dreadlord and you have to fight between gates or something for a while as in a small arena/ring.
-none of the villagers are protected.
-I suggest different colours for pings of main quests and optional ones.
-just a Knight guarding the entry to the warlock remains!?
-enemy units are repetitive. Add some spice man, some healers, some casters, some archers, some variation.
-pressing ESC brings out fade to black and back and gives the main quest message. I advise checking this issue for every map.
-"Wretched demon, you will...bla bla..." while he was sleeping

-what about a puzzle/mystery on how to get the orc bones in the cemetery?
-I'm not sure how and when the orc died but why wasn't he fully decomposed and how did he know of the place and the archmage?
-don't you think that Dark Ritual is a bit too much for 25 mana?
-again with the ESC thing, the lich repeated the line about the archmage.
-honestly, the lich's voice doesn't sound orcish but more like the undead necromancer (human).
-from acquiring the lich forth, the game should be harder with even more powerful troops.
-the Hydromancers only use Slow?
-I had me four zombie control groups and neither of them I needed.
-boss water elemental before the archmage or with him would be nice.
-the three elementals which were guarding the way to Ravenclaw, reappeared once during the battle with him. Maybe, more than once would make a better fight. I suggest they having different spells, like Bash, Critical Strike or even active ones.
-you should mention in the description what Finger of Pain can target. It does not affect heroes. What about structures or mechanical for instance?
-Thule is pretty much alone and weak if you use Infernal. By the way, haven't used that spell at all in this level.
The gameplay feels rushed. You basically kill stationary units on the map. It's plain boring. Too bad for the quite genius terrain. Have troops be strong, vary them, make it so that you need the zombies, not just to collect them.