orcishgamer: I own 2-3 copies of Sol Survivor, played a few levels and I have to say I'm doing it "wrong" or the game is insanely easy. Maybe it just has a steep difficulty ramp up later on, dunno. Defense Grid and a number of other TD games are far superior, which doesn't make it bad or anything, I just wasn't as engaged in it.
It does of course get a fare bit harder, though it is still an easy game. The thing that sets Sol Survivor apart from other TD games is the amount of different towers that you can build. Later on you get to pick between different "commanders", all of which offers a different mix of towers & powers, and the balance seemed to be pretty good.
Sadly, one of the downsides of this is that each individual tower gets less distinct.