These give off blue power-up tokens, which are used towards selecting one of many weapons. There are 4 front-firing options (guns, plasma, magma and laser-beams - guns can be built up quickly but lack top-end power compared to the others) as well as speed-up (only requires one token), side-shot, up to 3 homing missiles, build (a quick super-destructive blast that only functions when you aren't using auto-fire) and stealth (a period of invulnerability).
Each level has a very different setting, and an end-of-level boss to defeat. The chance to resume from the level you died at is offered up to level 3.
Unusually, the game has an option to emulate an auto-fire function using software (this makes it one of the few Amiga shoot 'em ups you can hope to get as far through on an emulator on keyboard as on joystick on the real thing).