Keep in mind, if you run out of money, go to the first area and fight mobs...
That being said, I thought the ending music was rather cheesy after all the hard work I put into the game...
Not sure how lower gravity would let a person do a double jump... maybe he's grabbing the wall next to him and launching himself higher? Oh well, it's just a game. :-)
Using the first Commander's turret option doesn't work too well against the ground troops, because you usually can't see too well over the edge of the TALL building you're on.