(This is in addition to my previous post)
I got past level 21 here, but I had to use my grenades to kill the creeps before they got to the end of my turrets. Weird bug.
so, played for a few hours. got to space and thought "alright, almost to mars." saw how far i still was. well, it was fun until i saw the real grind behind it.
 "SAS Zombie Assault TD"){kind=icon}
 "Into Space 2"){kind=icon}
 "Learn to Fly 2"){kind=icon}