So, this episode.
I'm not really sure how to write this review, because I don't know how to get my head around everything that happens. I will start off by saying this. The structure of the episode was seriously impressive. It was complex, twisting and turning much like the castle it was set in. I think we can safely say that this shows how far Doctor Who has come from the "kids science fiction show" Sidney Newman originally thought up.
And in the most part that is what this episode is. A very complex, very clever and very dark wade through the Doctors eternal quest to punch through a wall of a substance 20x hard than diamond. The set up worked perfectly and once the reveal dropped explaining the mysteries of the castle (the stars, the clothes, room 12) it really hit you how well it was written. And Peter Capaldi manages this episode perfectly as well, considering it was pretty much him for the entire episode, barring two/ three lines from Clara. He seems to work best when doing extremes of emotion, so this episode gave him two to go with and he went with them, running off into the distance.
And then we come to the end.
*strokes beard thoughtfully*
Well isn't this an interesting development.
It's interesting that they're going to base the series finale around something that was only ever mentioned before in the Eighth Doctor's TV movie and that was kinda swept under the rug afterwards by the books and relaunched series. I'm not sure about how I feel about it, but it'll be interesting.