My story is set in a fortress town built below a mountain, and revolves around the struggle between the witch who protects the mountain and the tyrant who seeks to plunder it for resources (think Princess Mononoke) It's primarily a fantasy story with some science fiction elements thrown in, particularly the villain's army of brainwashed soldiers with laser guns and stormtrooper suits, battle tanks (reference), and vehicles for drilling, chopping down trees etc. (reference). The world is also trapped in a perpetual winter, so snowfall is pretty heavy on the mountain.
At the beginning the antagonist's forces have been trying for years to conquer the mountain, but have failed due to the harshness of the terrain and attacks from the mountain's animals and witch that protects them both. I'm having trouble coming up with all the specifics of why the mountain remains unconquered.
What aspects of mountain terrain in an eternal winter would make it hard for vehicles and footsoldiers to successfully attack/plunder the mountain?
What type of vehicle would be able to advance up a mountain that is untouched by human hands and covered in snow that never really melts?