IIRC, the empty dungeon was one of the bugs reported in the German version when it was released. Apparently it has not been fixed yet.
You need something to take out the forge. On the first level of the prison, in the room east of the kitchen with the 4 guards, there is a book that describes how to make gunpowder...
Is that the one with the locked door? How do you open that door?
I have the three ingredients for the explosion, but I never read the book so I dont know how to do it, do I have to read it to trigger the ability to destroy the place, or is there some special way to combine the sulfer coal and salt without having to read that book?
Figured it out! Its coal on salt = Coal+salt mixture, then that on the sulpher to make the explosive, dont need to read the book to do it either.