主动防御(Active Defense)

Screw, wrote a detailed review, and it vanished on me. Cheap, basic, fun. No pathing, no fluff, no tactical options. Towers level twice without any choices to be made, so it is dirt basic. But the maps I have played on are interesting, and there are flying units which make tower placement more interesting. Paid less than a buck though, and I have gotten my moneys worth, so an easy thumb up, looks like there might be a lot of game here. ADDENDUM II: Thirty levels, plus endless, and ten guns total. About six hours, start to finish. Oh, and endless mode might just be actually endless, as you seem to be able to give yourself extra lives as much as you want. I'm done, but it was certainly worth the price.

Reasonably cheap and simple, but will keep you on your toes.