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I followed the suggested strategy with my hero, capturing the northeastern Castle on day 6. Since I had largely forgotten the map, I spent a long time searching for the exit from the dungeon leading to the second enemy Castle. When I finally found it, the Castle's defenses were quite strong, although passable. I decided not to waste precious troops on a siege and continued clearing the map, focusing on capturing the southwestern Castle. I stationed Dryads, Dancers, and Ents with Unicorns in the Castle, while I roamed around with Elves, Druids, and Archdevils, along with Nightmares and Succubi. By the way, I got lucky at the very beginning of the game; I found the "Armor of the Forgotten Hero" artifact in the sea. I was amazed; it costs around 20 gold in the artifact shop, but I looted it from the sea!
In the end, I besieged the last Castle in the final battle. I lost about 10 Archdevils, 20 Nightmares, and 80 Elves. I achieved this result after 3 battle restarts; in the first 2 attempts, the losses were much greater. Of course, these losses were no longer significant, as the red player had already been defeated, but I just found it more interesting this way.
Also, I didn't need to visit the Time Obelisks much during the mission, so I collected the remaining dragons at the very end