OK so y'all wanted an update... alrighty then.
The first four levels are just about complete background-wise; just waiting on some sprites for the stages now. Scoring system is almost fully implemented... just missing one detail to that. I am about to switch from snd.c to Squirrel 3.0 for the sound effects; I think that snd.c may be one of the current bottlenecks, and Squirrel is much more mature anyway. There's a little bit of occasional slowdown when playing 2 player, but it's not nearly as bad as some of those SNES shooters constantly had with only 1 player.
The game is challenging, but not over-the-top difficult. It can, of course, be customized to be more challenging, but by default, it's not too much of a headache. By default, it is probably about as challenging as Air Zonk's "Spicy" mode.
I may do more gameplay videos at some point, and as soon as we finish populating levels 3 and 4, beta testing will begin.