oh thank you so much for your feedback.
I am going to ad three more modes:
competitive: like you said, shoot common targets (invaders, aliens or something like that)
co-op: play together against aliens :-)
arcade: like the normal game but u have to beat diffrent genereals (cpu).
I like the idea of giving the towers its own health. but if u have enough money u can rebuild it after it went down.
And I am thinking of giving the speed-gun a stun-effect so you can disable the enemies defense.