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

I bought heroes during the process of capturing castles, so by the end of the mission, I had 4 heroes along with Arnel. I sent assassins to kill the demoness and destroyed the portal with Arantir's spell. After that, I placed all 4 heroes near the columns, and it worked. The ghost let me pass. After passing the guards at the bridge, following the advice of one of the forum users, I went to the left, where there was indeed a purple outpost and a port. After passing through the port, I received mummies, death knights, and zombies. With such reinforcements, I easily defeated the Castle of Holy Flame. That's basically it. :D