If you have problems to get that achievement.
How to Get What Happens In The Dungeon, Stays In The Dungeon Achievement
Before you give the money to the Guardian upstairs, go back into the Room with the mirror and switch to chanel II. Now turn up the volume. The Wizard will now become angry.
After that go to the dungeon and klick on it. After that scene go upstairs and throw the money on the stairs (2x) and pull the rope.
After the run down the stair go back into the mirror room and leave it right away.
Now go again to the dungeon and klick on it. You get the achievement.