Progress Update:
So I've got a main game-loop in place. Basically, you start off with 4 snowmen to choose from, and each time you complete a level a new random snowman gets added to your selection. When you start a level, you may only bring 4 snowmen with you. There is a system for "special" levels, and I'm thinking that these give you better snowmen, or something along those lines.
Also changed some textures and did a lot of back-end stuff (fixed map generation, level generation, animations, etc...). Also added a main menu, spruced up the UI, and added sound!
Also, the level generation is a lot more controlled now. It's still random, of course, but I can influence what kind of setup the enemy positioning has (for example, tanks will generally be in front and shooters behind).