On the plus side, origional theme, I like the winter concept w/unique tower types. I'm not a big fan of the upgrade system though, takes to long once you upgrade for it to take effect, plus because each upgrade upgrades all towers of a certien type only, it forces you to use almost all the same tower type to make it worth while, which takes away from being able to use a wide variaty of towers. Spells are a fun addition, although a slower cooldown would be nice.
As a side note, a plot would be fun, given the winter theme you could have creeps trying to invade santa's workshop or something <- just a random thought.
Overall I like it, and with a few tweeks it would be a great game, keep up the good work!