If by secret you mean you had to actually pay the bare minimum of attenti𒊎on to the story, then sure. "Secret." I don't think most people regard any of this as a secre🐬t. That said, I suppose it's notable that this predicted a sequel and... it seems we are basically getting that.

[spoilers, though you pro🤡bably shouldn't read this guide if you've not reached this point]

It wasn't Nebakov Fortress where you have your final confrontation with Istvan. That was Trosky Castle. This occurs after being knocked out at Nebakov, but you are escorted to the dungeon𒁃 at Trosky. There's a brief scene indicating such, which also explains why Capon isn't with you.

Though even if there wasn't that scene, it is clearly Trosky Castle just because you have to g🐼o through both the Crone and Maiden towers during this mission.