Wait what? That's it? Then how about the death mystery?
There are two deaths and one escape. If someone is locked in a sealed castle, they are going to die from illness, malnutrition, or natural causes. The one who escaped figured out the lattice trick. Anyway, the castle has been around since the Meiji era so even without the prison, having 2 people die inside is not unusual.
The current head of the family has never gone inside the castle, so he knows as much as any random dude on the street about the castle.