There was a moment from an episode of Doctor Who from a few years back which I couldn't help but be reminded of as I watched this episode. It was from, I believe (though, I could be wrong), Matt Smith's first episode as the Doctor - and, it was a moment that was intended to give us a glimpse of how the Doctor observes the world around him. In this moment, we had time seeming to slow down around the Doctor as he observed his surroundings - and, we saw him picking out details that were too small, and too far away, for any ordinary person to have any real hope of noticing them. It was a pretty great moment, over all - one that did a remarkable job of showing just how different the Doctor truly was.
The reason why I bring this up, now, is because this episode does something very similar - just with more of a creative flourish. Much like that moment from Matt Smith's time as the Doctor, this episode gives us a very interesting internal look at how Peter Capaldi's Doctor goes about solving the problems that confront him. Here, we see that, in times of very real danger, the Doctor will actually imagine himself safely back in the TARDIS - his thoughts moving faster than the action taking place around him as he tries to work out some way to save himself. It is another fascinating display of exactly how different this centuries old alien figure truly is - and, the sight of the Doctor safely in the TARDIS and talking to himself (or, an imaginary version of Clara, in this case), while the 'real-world' danger plays out in slow motion on a screen, was a creative way of visualising it.
To further complicate matters, it also seems that Clara is truly dead - her own attempts to interfere with Ashildr's plans in the previous episode having back-fired in a truly tragic way. While I suppose I should admit that I did find the scene of Clara's actual death to be a bit underwhelming, the very real sense of the Doctor's anger and grief that we get, here, goes some way toward making up for that. Clara does make a brief appearance, here - though, only in the Doctor's imagination. And, despite her pleas that the Doctor not concern himself with getting revenge, it seems clear that this is exactly he has in mind when he first arrives.
The idea of focusing an entire episode around the Doctor might have seemed risky, but this episode proves that it was a risk worth taking. Peter Capaldi did a fantastic job of, essentially, carrying the entire episode by himself. To be fair, though, the impressive set-design and the simple, though effective, design of the creature did each play an important part in establishing the tense and somber atmosphere - but, with a less compelling performance to serve as the heart of this episode, the whole thing would have fallen apart fairly quickly. On top of that, there are also some great revelations about what the strange tower actually is, and why the Doctor is trapped there - but, of course, that's all tied up with a fantastic climax to the episode that I don't want to spoil.
Honestly, this is probably as close to perfect as an episode of Doctor Who has got for some time. Sure, there are potential issues that might crop up when you begin to question some of the logic behind what you have just seen - but, that seems to have become a fairly common occurrence with Doctor Who over the years (it might even be a part of the show's charm). The episode, itself, is tense and dramatic - and, it's carried by a fantastic performance. Best of all, it also sets the stage for a, hopefully, equally fantastic final episode for the ninth season next week.