"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

I've only bought two heroes so far – I'm short on funds. I think it's never too late to buy a third. I captured three castles. I went looking for a fourth, following the trail of an enemy hero. But then, two demons suddenly appeared, very close to two of my captured castles. One castle is empty, and the other has Ornella with a small army. I had to abandon the search for the fourth castle and deal with the demons. Luckily, instead of heading for the castles, they moved towards each other. Now they can't escape from Arantir. Besides, he has a portal to the city, just in case.