I got pretty far, and then came back today and everything was gone. I did rate it five stars which changed the rating by .20 but now i am rating one star until there is a save option.
Private: General sir, can't we just go around the back and kill him? General: No Private! stop thinking, ideas like those are crazy and will just get you killed! *Sends full frontal assault towards alien, kills thousands of men* General: uhh...
You should make it multiplayer so that people can verse each other, this is what I had in mind. Both people have their own tower slots and can upgrade their towers to maximum upgrades, each person automatically sends out a basic wave and the waves get harder and harder but you can also use your money to send out additional enemy units and who ever survives the longest wins and you should get a ranking points to rank up.