These story levels were originally intended as DLC. I think they are far better than most of the levels in the released game. Alas, this DLC never got released. It features cut scenes that are as good as finished, has unique weapons and enemies, and an epic boss battle at the end. I designed most, if not all, of the hazards in these levels. Our art department would also often write down ideas for the levels, which we would sometimes elaborate on to make new obstacles. The tree that drops snow when struck, is one of those hazards. It can be very helpful to freeze enemies and shatter them in an instant, but players must be careful not to get frozen themselves. I also love the snowman prefab. Most of the snowmen drop a weapon when destroyed, but are completely harmless. But on occasion there is a snowman that spawns an enemy, to scare/surprise the player. You can see all of our prefab prototypes as well in this video.
The way we designed these levels is quite different from how we designed earlier levels. What we did, was simply design a lot of small bits and pieces of game play. This allowed us to pick and choose the ones we liked best, and then stitch them together to make a whole level. Of course this meant that a lot of designs got tossed out as well, but we also didn’t spend a whole lot of time on them either.
You’ll probably notice that a whole lot of weapons and enemies have a long string of weird text attached to them. This is because their descriptions were not implemented yet at that point (and they never were).
My only real gripe towards this approach of level design, is that it seemed we once again fell back to making a ton of unique backgrounds. I prefer laying out levels in the form of a circle, or a zig zag pattern. So you get to use earlier parts of the level as background for later parts. I simply find that more cost efficient. For a good example of that, look at the Demonbreed level for Left 4 Dead 2 that I made.