Ok this is what you've all been wating for a new kind of shooter. New stuff added. 2 player mode, shop, submit your highscore, weapons, enemies, diffrent platforms, cheats which you should try, and 11 stages plus 2 more for bosses. Oh and theirs also a special level after credits only if you beat the game, its just for like bonus points, it has nothing to do with the story.
If you can't beat the game make sure you use the cheats.