Daedalus pretty much nailed the tech details of the screen display setup. I'll do some blog post at some point going into more details about all the tech and optimisations in play here but at a top level.
Targeting A500 512kb / 512kb
Adaptive particle engine for the A500 - defaults to 800 but when a frame drop is hit it culls a stack of them so the game doesn't slow down. If the frames have been running fine for 1 second, it bumps the limit back up. If you have a faster Amiga, you'll be able to have thousands of particles running.
SFX - running Roondars mix engine in 4 channel mode leaving three channels for music
Music - using Leonards Light Speed Player for three channels of ProTracker but currently integrating my loop mixer so in effect the music can have 6 channels
Loads of cool Easter Eggs for the Amiga version.
Loads of control options - it's a twin stick shooter after all.
Achievements!
I've been doing dev streams on it every Wednesday night on
http://twitch.tv/djh0ffman for about 6 months or so. Starts at 8PM GMT if you're interested.