You win if you vanquish all the enemies. Take care of the mermaid, she must survive. So do you, so take care not to blast yourself out :)
If you're stuck, check the video game guide.
A level editor is available from the main menu and you can share, online, your creations with other players.
'1' to select the cannon
'2' to select the rifle
'3' to select jumping
'R' to reset the level
'N' toggles music
'M' toggles sound
'J' toggles collision boxes (editor-only)