To beat the 40 levels campaign: Buy a shotgun ASAP. Once you have it buy (top to bottom) missile, laser, missile. Max towers. Buy repairman and gunman. Start reading a book while your defenses take care of everything wihtout you shooting a single shot
A great phisycs system. Great feeling on turns. Perhaps instead of buying predefined upgrades, the game could give the player a choice on what to buy. Perhaps with the actual part names, such as suspension, transmission, etc...
Deafninga, you are playing online, if that doesn't make you a nerd, you are at least a geek in more than one level... stop denying your nature and embrace it!!!