Not bad. Simple. Make it get harder and make it so that the green dots don't spawn over the red ones.
Maybe add some sort of invincibility power up or something else to make it more difficult.
3/5 as it is...the puzzle is addicting(I've played puzzle quest for hours), take some suggestion such as mana bonus for 4/5 in a row. The towers need some adjusting and the graphics need to polished. Take care of that and it's an easy 5/5