One of the greatest strengths of Doctor Who, and the biggest reason it has endured so long apart from its ability to recast the lead actor, is its ability to slip effortlessly from genre to genre. This may be one of the reasons that the series has become more popular after its relaunch; in many ways, the show presaged genre-bending classics like 'Buffy the Vampire Slayer'. Changing from horror to comedy to action/adventure within a single episode or even a single scene was once considered to be a symptom of lack of focus--now it's the televisual equivalent of walking and chewing gum at the same time.
'The Snows of Terror' highlights the series' skill in an episode that is almost undoubtedly the high point of the story to date. Vasor the trapper is in many ways far more terrifying than the Voord, the Brains of Morphoton or the moving jungle ever could be, because the terror on display here is all too believable. When Ian leaves the hut and Barbara is alone with a man who introduced himself with, "I can break a wolf's back with my hands," it takes no great leap of imagination to picture this as a potentially terrifying situation. Barbara is locked in a room with a potential rapist and murderer, with nothing outside but endless icy wastes and the howling wolves. This is a nightmare made flesh, and the episode pulls no punches in showing it.
The first half, as a result, plays out like a slasher movie in miniature, 'Wolf Creek' in the mountains instead of the Outback, and even the polystyrene snow and film-insert wolves can't materially damage the creepy atmosphere. It's all too easy to imagine Ian's death as the wolves close in, and the much deeper and inescapable torment of Barbara as Vasor's captive. It is brutally effective, and the audience's relief when Ian rescues Altos and returns is palpable.
The tone shifts palpably after that, with Ian getting a great turn as action hero (again, it helps that this is an era when the show is obviously an ensemble piece rather than "The Doctor and Friends") and Vasor shifting from a wolf-breaking murderer to a shifty, sleazy, treacherous ally as we go to an Indiana Jones-style treasure hunt with more traps and mystical knights whose natures are never fully explained. There's a quick tour through the tropes of the sub-genre; rope bridges over bottomless gorges that break at the worst moment, relentless pursuit, collapsing tunnels, crawls over makeshift bridges, and "puzzles" that would perhaps take a bright child five minutes to solve. (The high point, to me, is Susan's reaction when the lashed-together ice bridge partially collapses. It's not the panic she's been displaying up to that point--it's a wonderfully put-upon expression that can be best summed up as, "Of course it breaks. Because what else would it do right now?")
If there's a single complaint, it's that Ian, Altos and Barbara grabbed hold of the Idiot Ball just long enough to not grab their micro-keys and travel dials before they went after Susan and Sabetha, necessitating a trip back across the gorge and back to Vasor's cabin. On the other hand, the episode wouldn't have felt complete without Vasor getting his comeuppance at the hand of the ice knights. No matter what tropes and what genres Doctor Who borrows, the series ultimately returns to the Manichean morality of the pulp adventure, and that means that even the minor villain has to be punished. It's the anchor that grounds the series, and allows it to effortlessly move between tones so easily.