Cheat mode:
Successfully complete the game on any difficulty level. Wait for the ending credits to complete, then select Start or Load from the main menu to access a cheat menu. Options on this cheat menu will allow (in order) the following: infinite lives, play as a monster, or level select
or

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

Level Password:
02 Up, Down, C-Up, C-Down, C-Up, Down, Left, C-Right
04 Up, Left, C-Up, C-Down, C-Up, Left, C-Up, C-Right
05 Up, C-Down, C-Up, Left, C-Up, C-Left, C-Down, Left
09 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

Flexible Nadia:
Start the game as Nadia and immediately hold Up, B, A, C-Left and C-Down.