This review contains some spoilers.
And now we come to the most unusual episode in this show's history - 55 minutes with pretty much entirely Peter Capaldi and just Peter Capaldi. There are certain actors I could watch doing their thing all day, for example Amy Acker (and not for perverted reasons) or Liam Cunningham. Capaldi would join that list but only with decent material... unfortunately, some of this isn't that decent.
One thing that actors playing the Time Lord have to become good at is talking to themselves - or rather to camera - in a fourth-wall bending manner. This is particularly required when doing Big Finish, but that's something rather different. Capaldi is excellent here; his Doctor is definitely in a dark place and not the one belonging to Garth Marenghi for that matter (oddly enough that's set in Romford). The Doctor has some mathematical calculations that would make Sherlock Holmes envious in this - this guy doesn't just jump out the window for the sake of it, he knows what he's doing. There's also a wonderful effects job with a skull that it would spoil to reveal.
The Maze, where nearly all of the rooms reset themselves when you leave them, is the stuff of nightmares. Seriously, you could make a good computer game with something like this. In fact, someone probably already has. One question that was raised on Gallifrey Base after this was about what would happen to all the excrement generated... something probably best not to think about it.
The Veil, with its attendant flies - and those don't tend to appear very often - is a chilling manifestation of death with some truly nasty hands; we don't tend to see the sort of injuries in Doctor Who (something that the Moff noted was an advantage of the 8pm timeslot - you can show blood) and when it's with one particular character, it's more distressing.
That said, this episode really, really drags. It raises some interesting philosophical questions... but could have done with being a good ten minutes shorter. That said, the ending, in which the Doctor reaches somewhere he's been trying to get to for ages (literally and metaphorically) is superb... with that final line even more so."Hell Bent" looks to be a very interesting episode indeed.
Conclusion
6/10