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Enter the house through the balcony directly north of Svartrau's West Gate.
Follow the Curious House down into the underground ruins to find Father Florin.
Say “Alright, let's work together” — he then explains how to get through the castle's side door.
Only what a source states. A quest with nothing listed is a quest whose rewards are unpublished, not one that gives nothing.
The side door is one of two ways into Greifberg Castle early, which reaches The Folk Hero without fighting a single officer. The other is looting the Side Gate Key from the miniboss Georghe, “The Rabid Duke”.
His offer can be declined and taken up later, as long as he has not been killed — which makes him one of the clearest published cases of an NPC's death closing a route. Note the spelling: this source and one other write “Father Florin”, while the walkthrough this site’s character record came from says “Father Floren”. They are treated as the same priest; the name needs checking in game.