I hear all this talk of drones being OP. I agree but disagree. I have also completed levels with just mines and just basic chargers and done just fine. I think the game is just a bit easy. Not in a bad way, more in a casual "It's just a flash game" kind of way.
Just found a bug. If you upgrade a tower under the effect of the buff spells (increase attack, range or fire rate) the towers stats are different than if they weren't. ie: fire rate is normally 26. Use frenzy. Now 13 fire rate. Upgrade tower making fire rate -2. Fire rate 11. Frenzy wears off. Fire rate 22. Would have been 24 if upgraded outside of frenzy.