Well, it really isn't an amazing game but it is everything it is supposed to be and nothing it isn't. It's doom invaders. Nuff said.
Siamorain it actually does make sense the lost souls would shoot lost souls because what do they use as a weapon in the game? Themselves! What else are they going to shoot? Now if they disappeared in the process, implying they themselves were charging on down, that would be more in line with doom but it would also just make the game too much easier because then very quickly the only rows left would be the cacodemons and the pain elementals.
And LaughingHyena64 the reason you shoot really fast when you shoot the shields is because you are only prohibited from firing the next projectile when the previous one disappears. If it makes it all the way to the top of the screen, that will be a long time, but if it hits the shield right in front of you, it will take very little time. Not very realistic for the firing rate of a gun to be the time until the previous bullet is out of play, but you see that's the way it was in the original game, as well as many other atari 2600 games like berserk, that was fun because you'd stand right in front of a wall (don't quite touch it because it will electrocute you) and it would shoot really fast.
