When you start playing Kingdom Come: Deliverance 2, Trosky Castle will loom above you, no matter where you are. It's almost always in the distance, beckoning you forward. If you go there to take a look around and wish to gain entry, you will discover that the castle is actually closed for business. So, how do you get in?

To complete Osina’s request, go to the privy next to the combat arena that you visited during the Opus Magnum quest, then interact with the stalls to reveal the demons. Since it just looks like a loo to you, go back to Osina and he sends you off again to investigate a mysterious door on the Maiden side of the tower.

You need to speak with three people about where the demons are coming from within Trosky Castle: Manyeta, Bertha, and the castle blacksmith. They can point you toward where these demons are appearing in Kingdom Come 2, and then you can sprinkle those locations with holy water, cleansing them.