I was a bit confused what the goal was while playing this. Obviously I can tell you lose at some point (so I guess "don't lose" is the goal?) but it didn't really feel difficult to keep placing buildings and shooting monsters, so I basically only lost because I got bored of playing.
Maybe I wasn't understanding something though! That could definitely be the case- my first run I missed the fact that you could move the cursor with the d-pad (all the other controls were listed at the start of the game, so I assumed that was all I could do!)
Still, cranking around to spin the moon was viscerally satisfying, and because the bullets travel in screen space rather than around the moon it was fun to spin a wall of bullet death around the moon repeatedly and rack up kills.