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I restarted the beginning twice. The first time, I explored the entire Dungeon, slowly building up my army. And the turquoise creatures also managed to accumulate... The second time, I went straight to the Dungeon, released Godric, and he immediately headed upwards with what he had, and the garrison at the gate joined him. That strong nuisance that appeared immediately and wiped everyone out in the previous attempt never showed up. Godric easily captured the Castle, while the mage continued to explore the Dungeon, collecting all the mines and other resource locations.
Added 9 minutes later
I also restarted the final stage twice. When asked if I was ready to fight Markel, he immediately attacked all three of us. By that point, Faidaen's army was not very good, and he was killed. The second time, I tried a different approach. Markel attacks everyone, regardless of where they are at that moment! I tried this: I sent Godric and Faidaen to the farthest Castles, where about 20-30 angels had already gathered. And they stayed there until the last minute, reinforcing themselves with all the available resources. The mage went to the enemy territory in the lower left corner and could pull reinforcements from the lower left Castle from there. There weren't any angels there yet, but the rest were enough. Then, I attacked the Castle with the mage, and Markel attacked everyone else. But they already had substantial armies, and they won!