"Ashes of the Fallen" - "Beasts and Bones" [Campaign 2, Mission 4]

Skull, I know that you don't need to capture the Castle to complete the mission; I've already played through it. I want to capture the Castle for the experience.

Added 2 minutes later
You need to destroy the Demon Castle, then go to the Dungeon behind it, and there will be 2 groups of demons. If you kill them, you'll destroy the portal.