Take a good look at your graphics. Creeps pass over the white part on the left, that looks horrible. Redo the green in the middle, and the part on top, on the right. If you do not allow towers there, you might want to add a bit of info about the creeps. Number the waves.
Try to balance your creeps strength, the amount of money they generate and the strength of the towers. I beat the game the first time round, and none of the creeps made it to the bottom of the screen.
Nice try, but still needs a LOT of attention.
 "Creep Defense"){kind=icon}
 "You Have To Burn The Rope"){kind=icon}
 "RetroShoot"){kind=icon}
 "Ultimate Sniper 2"){kind=icon}