Space Invaders Extreme Cheats

After clearing the fifth and final stage in the game during Arcade Mode (once you’ve defeated the final boss), the end credits will scroll. Following this, you’ll see that "Extra Mode" has been unlocked. Choosing this mode as opposed to the normal Arcade Mode will make each stage in the game tougher, as well as making the bosses more difficult to fight. In essence, it’s a more difficult version of the game that can be unlocked by completing Arcade Mode once through in its entirety.

To unlock the ability to play previously-beaten stages, complete that stage. After you do, you’ll have the option from there on out to play that stage whenever you want.