A funny thing about this game is that I started out without realising that I can move horizontally, which resulted in a better experience because I had to balance clearing the screen with dodging bullets and stomping on enemies' heads at regular intervals. However, if we take out that restriction, we're left with a run-of-the-mill arcade game that happens to have good art direction (excluding the final boss as others have mentioned). If I was to improve in one area, that would be the balance. There is simply no need to ever take Lives or Grenades when you can get Gun and Stomp, which are the two most used abilities and flat damage increases, which is a pretty big deal in a game about killing things as efficently as possible. Fix that and you have on your hands a decent game that doesn't set out to do more than it has to.