There are Secret Levels and Expert Levels hidden in the game.
Shooting Orange Boxes, Targets, and 1 ups while on instruction screens add more points and ultimately more coins to your total.
There are 40 normal levels of Astro Barrier.
Pressing "1", "2", or "3" before clicking "Start" will start you at a later level. For example, pressing "1" will start you at level 10.
Shots are not reset between level transitions, which means if you can manage to shoot on a level transition (either by using a blaster or by making so in a tutorial screen), you can make your shot last through to the next level.