Orignality!
Ahh what a refreshing feeling to see something new for a change! I loved theentire defence only dynamic, and the difficulty was just right, for me anyway. Things to improve on: not much really. Maybe a few new enemy types, and maybe a few more powerups, but I'll be darned if I can think of what you should add without changing the entire dynamic of the game. Oh and one little bug I think I may have found: Just when I started a level, I paused it. This prevented me from putting up any more defense screens, but the enemy was still shooting! Not sure if this was a one-off thing, but it may be worth looking into. A shiney new five for you!