There are 7 levels in level 12 after that there is LAST LEVEL 13-1 which is the last. You need to beat level 12-1, right through to level 12-7 then level 13-1 to finish the game if you lose your lives at any stage you will always have to start with 12-1 again you cannot start on level 13-1.
Hint try starting form the start and getting plenty lives as when you lose a life in later levels the level becomes easier i finished it with over 20 lives at level 12 onwards, and a score over 2 million - that is PC version only,, - i cant beat it on xbox 360 controls are too hard