An enjoyable episode with some funny moments and a great performance by its key guest star. Not one of the greats, but definitely a pleasant 45 minutes. The next time trailer also looks very interesting.
I'm going to have to start with the elephant in the room... not that you could ever compare the adorable Maisie Williams to an pachyderm. Arya Stark is one of the best things about Game of Thrones; it's become abundantly clear that her actor is definitely a name to watch and after that show (I personally hope that Arya's not going to end up having a death scene of her own... but you know what Games of Thrones is like), I'm sure she's going to do great things. She's only 18 and has already gotten more plaudits than actors twice her age.
As the Viking girl Ashildr in this episode, she is highly engaging in every single scene; especially a two-hander with Capaldi in which she demonstrates an almost Time Lord sense of her place in the universe. The reveal of her actual status is well done - and it's combined with an explanation of why the Twelfth Doctor has this current face.
Peter Capaldi gets a chance to portray the multiple facets of his Doctor - at times larking about, downright rude and distinctly brooding. It's very, very interesting to see that what was originally meant to be a gag about the Doctor speaking 'baby' (which I would point out is a 'language' common to infant Homo sapiens the world over) becomes the source of some key emotional scenes. I would however have to say that the use of "A Good Man" for key moments is a bit too noticeable.
I'll miss Jenna Coleman once she's gone from the TARDIS; she's proven to be one of the best companions of the last 10 years and in this episode, she definitely has some great scenes. In particular, the one on the Mire spaceship, where she gets pretty close to defusing the entire situation without any further fighting... only for Ashildr's big mouth to wreck it all. Sometimes sentient beings let their pride get in the way of their common sense.
The Vikings were generally pretty well done; while horns on the helmets is of course an anachronism (and a deliberate one), they're sympathetic people. Training a batch of hapless villagers to defend themselves is a rather old plotline, but it's done well here - I have to admit that 'Noggin the Nog' was my favourite Doctor-bestowed nickname. And managing to start a fire before the enemy even arrives... classic.
The Mire - good one shot villains whose reliance on technology ends up being their downfall; both the external suit and creature inside are very good SFX jobs. Their defeat did seem a bit tonally odd - "Benny Hill" music and all, but it was soon brought by a key discovery that shaped the final part of the episode.
The ending sees the Doctor make a well-intentioned decision that could have significant consequences... and one several people may come to regret.