The enemies fire faster the fewer there are, to keep the tension up once you start taking them down. I'm sure the timings could be tweaked to make this feel better.
Also thanks for the kind words :D
I guessed that was what was happening and I think it's a good idea. Perhaps the only issue is that it meant that the very first enemy you come across, from my personal experience, was harder to take down than the following 4, which as an introduction / tutorial piece didn't quite feel right. Maybe just that very first enemy could be slowed down to match the speed the following 4 will be firing at initially so that you have 1 easy enemy then 4 easy enemies which gradually become harder as you take them down, to really ease the player into it? It's a very minor gripe though, and quite possibly a personal opinion that isn't shared.
Ah I see, I think I just played it too much myself and I didn't realize the first drone would be quite tough for someone who never tried it before :)
What? No! I'm awesome at games and it was SO easy... Ok, I died a couple of times. :P
I think it was the fact I so frequently found my shields were depleted because there was no time for a recharge between shots. Yeah, balancing your own game which you've played over and over can be a nightmare, and you're almost certainly not going to please everyone whatever you do.