An exciting idea to say the least! Quite taken by it. One idea I have, though simple in nature, would provide a very fool proof, easy to implement yet interesting to see would be something like this.
Every level starts off with a simple percussion line. 4/4 kick/hi hat/kick/hi hat pattern. You know, the basic of the basic, foundational work. Doesn't HAVE to be this simple, but overall, nothing that gets too complicated. One of the levels I think should be very heavily percussion oriented. Every shot, every creature death, a percussion sound. So towards the hectic moments, you've got some brutal war drum patterns blasting away.
The premise behind the system is to have a kind of clock active in the background. A timer if you will. This timer acts as the number of shots each tower can fire, at set intervals. For example, a 4/4 beat means that you get 4 measures per bar.
The first tower might be able to shoot 1 bullet per 2 measures. Or in short, only in time with the kick sounds *all towers should fire at designated times in a measure, so it's not going to overcrowd and become annoying*
The second type of tower might be able to shoot 1 bullet per measure. So it fires off in synch with both the kick and the hi hat.
Third tower might be able to fire 4 bullets per measure, so give it a light hi hat sound. Small bell of sorts. The slower towers uses bigger louder percussion.
Then you can get funky with it. Perhaps let a tower first off 8 bullets in the first 2 measures, and then 4 in the next 2 and so forth. The more varied you make it, the more interesting the percussion beat is.
Since the level builds only the percussion it allows the musician to still go nuts with the melody. Unrestrained as long as it's appropriate to be accompanied by a heavy percussion storm *ie, not too much details on the instruments/synths, mostly leads, slight harmonies*
hope that wasn't pointlessly confusing.