This game is very difficult, I don't like how you have to stand still to reload the towers and how long it takes, makes it almost impossible to defeat waves without losing lives.
You don't have to stand still while reloading. Once you press the R button the tower will reload in couple of seconds. You can even carry the tower while it's reloading.
I second what Scurvy said, you don't have to stand still and be hit. The main purpose of the fireballs is to stop you from just standing there pressing the reload button :D
