This is probably one of the better game that's been submitted to ludum dare 25.
the concept is pretty unique and i think it's good that your still updating it after submission.
There were a couple of glitches though. First off after i got new invaders the old ones were still stuck
onscreen. Also the writing on the wall would sometimes stay there on new games. i also think it would help if you made the tanks blow up or something when you hit them.