Dantist
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First, I captured the first Elven town and the mines next to it, then I rushed headlong towards the Demon town. There, I bought two heroes, sent one to the Elven town, and the other to gather resources and capture mines, while the main hero just killed everything in the area. I exited the Dungeon in the lower right corner, then either traversed the map to the left entrance or returned the same way – I don't remember exactly. Of course, I was building up both Castles all this time. Next, I gathered a two or three-week supply in the Demon Castle and started roaming the Dungeon. I encountered dragons and decided that it was more efficient to kill them, even if it meant some losses, rather than waiting another few weeks. So, I killed the dragons and exited the Dungeon. I tried to capture a town... but it didn't work. Then I simply occupied the nearest mines, as the hero stationed there didn't attack, meaning he didn't even leave the Castle. At this time, two heroes appeared on the map: one in the Dungeon (coming from the eastern entrance), and the other above the Elven Castle. I had to teleport to the Demon town and chase him around a bit. But I was really worried about the Elven Castle, because I probably wouldn't have been able to hold it. Fortunately, the computer messed up and went not down towards the Castle, but to the left towards the entrance to the Dungeon. Although, it might have just reacted to my second hero, who was hanging around nearby. Anyway, I killed both of them, replenished my supplies in the Castle one more time, and headed towards the eastern exit of the Dungeon. From above, I found the second Elven Castle and captured it without much trouble. After that, it was just a matter of technique: I built up my towns, patrolled the map, killed a couple more heroes, gathered an army from the two Elven towns, and then captured the third town.
P.S. Be sure to build Mage Guilds in all your towns and definitely level up Chaos magic. Without Earth Shock and Meteor Shower, it's very difficult to continue playing, IMHO