Play Brain, The Online

This is a side-scrolling shoot-'em-up, in the style of Gradius. You play as a helicopter gunship, and when you enter a building as a spaceman(who can shoot in eight directions).

Interestingly, you can manually scroll up and down as the game scrolls right, but cannot scroll backwards. Destroyed enemies leave power-ups (improving your firepower), smart-bombs which trigger on collecting them, and floppy disks which you must collect to win the game.

Another factor is your oxygen level, which shows as an air meter at the bottom (you can gain more air through each stage). There are end-of-level bosses.