Great concept/idea, but little depth.
I like the whole idea of building turrets that shoot things that run down a lane, but this is pretty basic.
The turrets, for example, are pretty linear. There's no altering paths one would take; Just build enough power plants to match so it matches with the rate you can build turrets. If you introduced different types, such as missle turrets or laser turrets or whatnot, that'd be awesome.
Another thing I'd like to recommend you doing is put a lower upgrade cap on turrets, and the ability to upgrade multiple turrets at once. Having one turret upgraded to 30 will overwhelm ANYTHING, and it's way easier to get them.
All in all, very fun, but you should tweak it a bit. Add more levels and structures and junk.