Nightmare Creatures 

Cheat mode 
Enter C-Down, Up, C-Up, C-Left, C-Up, Left, C-Down, C-Left or Left, Up, C-Down,
C-Left, C-Right, C-Up, C-Left, Down as a password.
Alternatively, enter C-Down, Up, C-Up, C-Left, C-Right, Left, C-Left, Up as a password
then select "Start" from the main menu to access a cheat menu.
Options on this cheat menu will allow infinite lives, inventory, 
continues, play as a monster, and level select.

Level passwords 

Level           Password
2               Up, Down, C-Up, C-Down, C-Up, Down, Left, C-Right
4               Up, Left, C-Up, C-Down, C-Up, Left, C-Up, C-Right
5               Up, C-Down, C-Up, Left, C-Up, C-Left, C-Down, Left
9               Up, Right, C-Up, C-Down, C-Up, C-Up, C-Right, C-Right
11              Down, Left, C-Up, Down, C-Up, C-Right, C-Left, Up
13              Down, C-Left, Right, C-Down, Up, C-Down, Down, Up
14              Down, C-Down, C-Right, C-Left, C-Up, C-Up, Left, C-Up
16              Left, C-Left, C-Up, C-Left, C-Up, Down, C-Right, C-Left