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I played on "hard" difficulty. The overall tactic is simple and has already been described here. First, we clear the territory around the Castle, then we go down and capture the Fortress and the Crystal Mine (I didn't capture it right away, which caused a lot of difficulties), then we take another Fortress at the bottom, buy a 2-week growth bonus, and head to the basement to fight the demons.
We systematically clear all neutral creatures (trying to avoid losses, if necessary, we bring them back to the city), except for those at the crossroads (it takes too long to go around in circles there), we capture the Fortress in the north of the underground, and along the way, we defeat enemy heroes. Then we try to break through the enemy in the Castle (most likely, it won't work). If it doesn't work, we go to the guy in the center of the basement, who gives a Llama for a staff - this is where very difficult battles take place, as there is simply not enough time to heal our units; only summoning Dark Elementals + summoning Water Elementals saved me. We bring back the lost undead in the city and build Llama in the city (although they are not very useful). We teleport to the Castle (it doesn't matter if the enemy has captured the Fortresses in the basement). Here, we try to buy back EVERYONE and the GREYHOUNDS, paying special attention to powerful stacks of Liches, Spiders, and Ghouls. We go to the underground again, preferably as soon as possible. Now the goal is to capture the city. Ideally, if the demons haven't had time to recapture the Fortresses, then everything becomes easier - a lot of units are just walking around in the field. But we defeat them IN ANY CASE. Brief tips for battle tactics: we pay attention to the most powerful stack and bombard it with Liches + Lightning. We also summon Elementals and place them on the outside of the wall to destroy the nearest archer. Our priority is to heal Liches and Spiders (mass healing + necromancy). When the enemy comes out of the Castle, we use mass dehydration, as all units join the battle. As a result of this battle, I had 20 Specters left. In the Undead Altar, I bought EVERYTHING I could - ALL Liches, Spiders, and half a stack of Ghouls. We quickly capture the Fortresses in the basement (Nastya's magic helps even with a small army).
Then we go to Lyudmila, take Vampires, and guard the surroundings (an Orc and a Necromancer will come, and the latter usually runs away). We defeat the Orc and easily take the Orcish Fortress. As a rule, no further problems arise - we defeat the Necromancer, sail by boat towards the lower left corner, and take two Castles. We spend a couple of weeks building up our army and level up Nastya by fighting neutral creatures (they give a LOT of experience). I won the battle against Miranda with an autobattle, losing about half of my army, but my losses were greater. There is nothing difficult in the battle.
The main problem was defeating the demons, especially when I messed up with the crystals and couldn't build Spiders for a long time. As a result, I barely managed to win the battle against the demons in the late game with four un-upgraded Spiders (which is very few), but after the demons, it is no longer possible to defeat Nastya on the map. Regarding skills, from what I remember: 3rd level Dark Magic, summoning Dark Elementals, mass dehydration; summoning Water Elementals; mass healing; logistics, 1st level tactics; mass haste. I maxed them out. I didn't play with Lyudmila AT ALL, I just collected resources.