MJPollard said:
Hadn't thought of that; it's a good possibility, too. And the ringing of the Cloister Bell doesn't necessarily mean "catastrophic, universe-ending danger" in this case. Remember, the whole hotel setting was unreal to begin with, so the bell could've just been "atmosphere" to underscore the Doctor's deepest fear.
It could also be the Tardis exploding destroying the universe... it would be horrific if it showed him this whole series isn't real but is taking place as the Tardis blows apart part of the way into The Eleventh Hour and 'his room' is really the way out to real, reality.