I love this game, and I finally got the real ending! how I got there was a bit surprising, but fun nonetheless. I only wish that there was a secret, SECRET ending. I love that in the first few levels you get new powers, and use them to unlock the rest of the game. I also want more levels, or at least a sequel.
And now, to help the poor, helpless souls who have no clue how to get to the secret levels.
0: try to get the power of level, and change the level to 0. you will get a power important for the rest of the secret levels.
-1: use 5 1-ups on the plant in level 1. then fall in the hole.
-2: in -1, have the music turned off, the enemies dead, no 1-ups remaining in the level, no 1-ups left in your stash, and get hit by the spikes.
-3: turn on the music in -2, and use the 1-up when all enemies are alive. get the 1-up and reset the level to do this easily.
-4: in -3, turn the music off and reset the level. then, use your score and get to the enemy.
-5: after reaching -4, go to level 9 by going through the doors in the negative levels and use your score to go through the gap at the top and get to the door outside. it's that easy.
I hope this review helps you if you're stuck on the secret levels.