I know it's rather late, but I want to say thanks to Hiino, Deformed Knee and AI for taking the time to reply; it's great to hear your feedback, and I'm glad you all enjoyed the experience.
I like a challenging level myself (as long as it doesn't cross the line to frustrating) but I also want to play to the audience; I don't want to alienate those players who like their challenges less demanding. This level is kind of border-line as hard as I can reasonably make it I think.
This was another level where I looked at the passage of time and weather. When you first emerge from the lab, it's a misty, murky night. As you climb the mountain across the valley, snow begins to fall. A crevice in the mountains just before the ruins show snow settling on the ground, and when you emerge from the ruins it's daylight and very snowy. For me, that sort of movement gives a level a sense of progression and time, and a greater feel of realism (which matters to me even if the concept of the level is essentially rather silly).