Poor Klaus... I'm going to feel bad if he gets flogged.
And he probably will.
See, I told you we should have busted through that secret door
The adventure comes right out and says 'don't let them find the secret door, and even if they do don't let them open it!' Obviously I'm much more reasonable than that (Julian would easily find it anyway), but it's still a dwarf-made stone door that is designed to be opened from one side only. It would take ages to break it open, and might need a team of engineers to do it. Or a bomb.
Plus you can easily find out which building is above the cellar, by having a look from the street.