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I passed the outpost leading to the teleport on my first try, and immediately encountered the main hero of the yellow faction. Luckily, I managed to reach the waterfall and double my mana; otherwise, it would have been game over, because after traversing the border guarded by 7 hunters, only 2 remained, and I had no mana at all.
(If needed, I saved the battle replays.)
Now, exactly 6 weeks have passed. I've destroyed another strong outpost and am on my way to the yellow castle. After defeating the main hero and his entire army, there shouldn't be any strong garrison there (one, at best - two). Next, I plan to deal with the green faction, although I might try to defeat it with my second hero, who has my main army.
I'm postponing further progress until next week because I'm going to the forest for the weekend.