Worshipper – “Conquest” [Campaign 2, Mission 3]

I'm completely stuck on this mission; I've even lost the desire to play. I successfully capture two elven towns: the one near the garrison and the one to the west of the center of the map. But then, an enemy hero appears with an army as if he has not one castle but five. He defeats me in about ten turns with minimal losses. And, to make matters worse, in the Inferno castle, you can't build either a castle or strong monsters. And the army I manage to raise in the elven towns is still twice as small as his.

What am I doing wrong???