So the Heaven Sent castle is designed to progressively scare the Doctor until he would give up the Hybrid's identity. The logical assumption is that there is someone watching or at least listening (at the end of the episode, the Doctor remarks that there's probably someone still listening), because, well, it's an interrogation.
But if that's the case, shouldn't the watcher have noticed what the Doctor was doing? Shouldn't they have realized that the Doctor was slowly escaping the place like, billions of times over?
Why is no action taken? Surely after a few million times the watchers would understand that there is no way he's going to confess. Why was he allowed to continue?
I imagine we'll need to see the final episode for a proper answer.