Darrol Blake's decision to shoot the location scenes on videotape help blur the lines between which is location footage and which scenes were shot in the studio. A key example of this is the scene that almost sees The Doctor sacrificed in the stone circle. It's so expertly lit that it does look like it was shot on location. The only thing letting it down, though, is the obvious use of a day-for-night filter that Blake had to use on location in the absence of a genuine night shoot.
One of my all time favourite Tom Baker moments occurs in this episode. It happens in the scene where Romana is outlining her plan to save the damaged K-9, when, all of sudden, Baker bursts out "Brilliant!!!", then looks almost shocked that he did such a thing. Pure hilarity - I laugh, and have laughed, at it every time I've seen it.
With such a tiny cast of characters, and with two of them (De Vries and Martha) getting killed by an Ogri in this episode, the mystery of who the Callieach really is isn't actually much of a mystery. Thankfully, we're not kept in false suspense for too long before Vivien Fay is revealed as the culprit. It's at this stage that the story looks to be taking a drastic turn from Hammer Horror into the strange, science fantasy world that Doctor Who has found itself in for the past year and a half when Vivien zaps Romana off to wherever with some sort of laser staff...
And speaking of Romana, and forgive me for making this sound like a "rate the female companions base on their looks" contest, but Mary Tamm looks absolutely breathtaking in her red dress and with her hair down in this episode. Romana was my first companion, and she is still, to this day, my vote for the sexiest companion in Who. There - Maxim article over with.