Try this!:
Easy way. Hard/Green lvl. 1)Buy three TACK place on inside if very first turn. 2)Upgrade for speed. 3)Repeat on turn 2. 4)Buy 2 CANNON place on inside of turn 3(near middle of board) and upgrade damage all the way. 6) Keep up that pattern.. 3 TACK 2 CANNON around the board. When you are about half way filling the turns upgrade all towers full. Then just keep adding more