Off to very good start. I like the feel of the user interface and the nice smooth movement of the enemies and how you can run multiple waves on top of each other.
some general comments (i played normal mode):
the weapons don't "feel" all that different. like the splash damage one -- i don't see it splashing.
sometimes my towers just stopped attacking creatures, i wasn't sure why or if this was on purpose.
when clicking an existing tower, it would be nice to remind me of its characterstics (like when you click on new tower icon).
one of the towers said "magic damange" -- i had no idea what that would help do.