A feature request: Have the engines turn off if your ship is upside down. When I hit something and tumble, if I hold the down key while trying to recover the buffer gets full a its impossible to control.
Sparta, No Shots Fired: For those who feel that guns are too violent - buy and max out a Flame Thrower and 3 fans. BBQ! Lazy? Use a maxed Flamer Joe (centered) and 2 Fans with a 400 wall and repairmen.
@aquatorrent and others playing the campaign: You get more money for killing them early, and almost nothing once they reach your wall. So either be a perfect shot or alternate killing the lead guy with one that just appeared. I really don't know - the reload delay always puts me behind so I get less money, then I don't have enough firepower for the next wave.
I see comments about selling/moving towers during a level. I find the recovery lag and the recharge timer too long to risk it. Only on the two-path level, the last level and one other did I dare even try it. My timid solution was to come back to hard levels after I'd upgraded enough. So the game IS very well balanced. :D
TO GET CREDIT FOR the Badge of the Day - Return to your profile page. As @Meyismyname suggested, that is all I had to do. I suspect that game progress is stored locally but badge credit is kept by Kongregate.
FYI - After a tower kills an enemy, it won't see ghosts that's are flying over towers. When the ghost passes over open floor it can target it. Also, if a tower is built where an enemy is already in range it may fail to see it.
Your nanos that are traveling will eliminate red nanos they run into - so as soon as you're producing more than them: select several of your cells and click once on the four weakest cells, select other cells and repeat quickly. Send them to your own depleted cells too . Fill the screen with nanos traveling in every direction and the red nanos will never make it to another cell.