The game keeps breaking upon starting a new level; everything freezes, so I have to restart.
It can vary: sometimes it happens after the first level; sometimes it's after 5 minutes worth of levels; or sometimes somewhere inbetween.
Edit: I figured out how to fix it: just press [esc] and click "BACK". Ok, I easily finished in 2:40... much shorter than I expected. Melee is op with the upgrades; I finished with an excess of like 1600 points and never used a heal, gun, or teleport.