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Finally, I got to this mission in version 3.0. After capturing the Castle, I saved the game. The auxiliary hero (Ilfina) immediately warmed up by the campfire, practiced on peasants, and looted the chest. Then, she reached the second campfire. I played for 3 or 4 weeks, and the purple hero just stood there, motionless. It was even impossible to attack him.
I loaded the save. This time, Ilfina sat quietly in the Castle like a mouse. And on the second week, the purple hero, level 19, attacked the Castle. Ilfina easily defeated him because Faydaen had left her almost the entire army. He only took the dragons and 6 dancers with him. When Ilfina got tired of waiting for the second enemy, she made a sortie. The peasants and the chest were gone, but the 2 campfires remained. As soon as Ilfina returned to the Castle, the 2nd enemy (Kaspar, level 19) attacked her. She leveled up to level 8. I don't know yet how the other purple heroes will behave, but the mission is doable.
The latest version of the bug fix tool fixes a minor bug in version 3.0 in one of the campaign missions of Lords of the North.
Added after 22 hours 35 minutes
Wendy, maybe you kept the hero in the gate? My Ilfina is in the garrison. She only runs out to the source after each battle to raise morale. Deidra and Kaspar attack the Castle in turn. At the beginning of the mission, they were level 19. Now they are level 23.
The enemy from the Castle immediately runs out to capture the mines and collect goodies. If you don't waste time collecting floating debris, Faydaen can capture the Castle before he returns and buy the troop increase on the 2nd week. The Castle is well-built with the hero. Therefore, there is something to buy. And the auxiliary hero can collect the debris.