Pretty cool!
All is good, I appreciate the system how you have to gather machine parts in order to create more towers. It makes it all more complexe and keeps you worrying about the game all the time (or you'll loose those pieces....)
I'd say also that there should be a way to follow the achievements on each level, like for example if the level was cleared without loosing any life and so on. That way I could keep an eye on my own records and try to improve them :)
Good work!