Alright so Day 13(I reached mars on 11, Im not even trying to get it in 3) I have all the upgrades except for the lack of gravity, and im winning by NOT PRESSING ANY BUTTONS.
Im playing as i type this comment and staying at a reliable 60,000 feet.
This game is really incredible, the developers of this game, you guys have got to know, this is a Winning formula. All this needs is a little bit of expansion and this would be one of my favorite TD games of all time. Some extra towers, music attacks, and the like would make this game shine.
P.S. You also got the difficulty right, I beat the first 2 levels on normal and was thinkin this game was no sweat, i tried the 3rd level about TEN times, and i lasted about 45 seconds in my longest run. that means it was done right, Normal can be completed by anybody, but hard requires time and effort.