I take that back... lightning (yet again) seem to be better at lvl 5 vs two of them. Other guns like the machine gun and rockets though suck for the upgrade cost.
I think one of the big issues is that lvl 1 towers are all you need. Upgrading seems to do very little except increase the range. I know it increases the damage, but not enough to warrant doubling the cost of the tower, just build two...
Game needs a bit more balancing. 4/5 because it was challenging setting up a good strategy but it seems that the first 10 levels are crucially hard. After that it's a joke. I'm not even watching the game and haven't been for last 10 levels and it's fine on auto... I might stop it before the end and sell everythign for a few points heh.
 "The Great Siege"){kind=icon}
 "Orbital Decay"){kind=icon}
 "Frontline Defense First Assault"){kind=icon}