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No problem, the hero should have the creature summoning skills upgraded. I chose Air and Dark magic. You take a main hero with a minimum number of creatures and relentlessly advance towards the Castle located in the upper part of the map. Don't forget to attack bots along the way to collect resources. Two heroes with upgraded movement and resource gathering skills will collect them. Don't forget to replenish mana along the way. In the very end, or a little later, you approach the enemy Castle. The battle scheme is simple: send a Dark Elemental to the ranged units, choose whichever one you prefer, and there it is, the red flag – he's tough, and everyone attacks him. On the second turn, summon two Air Elementals under the gates. Then, I use Subdue, Chain Lightning, etc., and the Fortress falls easily.
And here, the most interesting part begins. Leave the hero in the Fortress; let him sit there and regenerate mana. There's also a flag there, and then dogs start coming from the Underworld. The scheme is the same: Elementals and finishing them off. But here's an unpleasant moment: two enemy heroes attack your Fortress, and you meet them with a purchased hero, upgraded for the same Elementals and magic. Don't forget to give him the Dynasty Staff; he simply tears apart the newly arrived hero in the Fortress.
Then, in the first captured Fortress, 4-5 enemy units will come to kill you, and then you yourself enter the portal. There's immediate mana regeneration there, and that's it. I think one Fortress is defeated in the mission. Then, you are thrown onto the surface and continue to defeat enemies. After that, it becomes less interesting because there are no more opponents.