Interesting how you can corral the enemies by clearing out the ice that they need to spawn on. It kinda ties in with the premise too, because while your planet is on its last legs at first, you eventually gain the upper hand against the aliens as you defrost more and more of the planet, putting you in greater control of the battle.
I must ask though, how do you win? The progress bar at the bottom made me think that once I cleared out all the ice, I just had to finish the wave and then I was done, but they kept coming. Do you need to both defrost the planet AND clear out all the waves?