Final Boss on last level:
Enter BLKX8 as a password.

Hint: Extra lives:
Move to the left edge of the screen instead of going through one of the two doors at the "Continue/Quit" screen that appears when a game is over. You will enter a secret room that contains four extra lives.

Hint: Pugsley's den:
When you first start the game, go inside, but do not climb any stairs. Go left so that you are under the door to a room. Push Up to enter a secret room. You may enter this room any time, but the extra lives will be gone until you die and use a continue.