To me this story serves as a perfect example of how to do a Base Under Siege story and why Troughton's era does them much better than other eras. It uses it's long runtime and the fact that it's largely set in the same few rooms to create a really eery atmosphere and sense of claustrophobia. The London Underground setting really helps add to this as most of it is set in small corridors with dark lighting (which almost distracts you from how unthreatening the Yeti design is). The score is probably the thing that contributes the most to the atmosphere. It's so creepy and all of these elements work together very well.

Another thing I really like about this serial is the characters. The return of Travers is cool and creates a nice though line between this and abominable snowmen as well as the return of the great intelligence. He has a daughter who is wonderful in this story. She's a scientist and gets to work with the doctor a lot along with delivering a good feminist message for the time. I also enjoyed the sketchy journalist character and this marks the first appearance of Nicholas Courtney as Lethbridge-Stewart. It's easy to see why he became a main stay because he certainly makes a mark here, giving that sceptical yet still grounded approach that his character would later do so well in Pertwee's era.

Overall while this story is not perfect (it does suffer from some pacing issues) I think it's really well written and the perfect example of how to make a tense Base Under Siege story. (Also for this rewatch checking out the animation was definitely... an experience. Will be sticking to recons in future.)