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Indeed, in version 1.5, this mission was made more difficult. Now, the garrison in Erivel constantly grows.
And while I was capturing the northeastern Castle, a hero with a decent army emerged from the darkness near the watermill by the river and captured the southern Castle. Luckily, a tavern cannot be built there. Later, when I went to retake the southern Castle, another hero with a weak army emerged from the same place and headed towards the northwestern Castle. I couldn't allow this – I blocked his path with a supporting hero and the weekly reinforcements from the northwestern Castle. Then he fled to the southern Castle. There, these two weaklings waited for Agrael to arrive. Gathering a sufficient army from the three Druid Castles to capture the underground Castle is just a matter of time. Since my Agrael reached level 24 in the previous mission, it didn't take long to gather the army.
After defeating the superior forces of the underground enemy, I became overconfident and decided to overwhelm Erivel. I stopped construction in the Druid Castles and lined up all the supporting heroes in a chain to deliver reinforcements to Agrael. Agrael, without regard for losses (attacking phoenixes and dragons with a small army), reached level 30.
When I attacked Erivel, I discovered that there were 40 Emerald Dragons against my 15 Archdevils. The enemy had an even greater advantage in the other units. But I had Agrael, level 30, against a dumb hero, level 18. Even without Chain Lightning, Agrael is Agrael, even in Erivel.
In my opinion, the computer controls the hero in battle more clumsily than the warrior. The battle lasted for 20 minutes. As a result, Agrael had almost all of his Archdevils, Succubi, and Sorcerers intact. The rest perished. The enemy stubbornly fired at Imps, Cerberi, Nightmares, and Demons until he ran out of archers and Sorcerers. Dragons and Ents under Berserk destroyed everything in the Castle.