Damn good, but needs more levels
I love "defense build" type RTS, and this is a very good one. This is one of the better games on Newsgrounds, and I could easily get addicted to it, but I have two suggestions.
First, the game really needs more levels, a lot more levels. I'd concenrate on the land caravan levels, since they seem to be the best written so far. The assualt mech level was cute, but not up to the standard set by the rest of the game.
Second, I'd put some sort of indicator as to how far you've upgraded a turret. I ended up just guessing which turrent was upgraded to what.