Go to the Main menu and press Minus + Plus to display the cheat screen. Now, you can enter one of the following codes to enable the desired effect:
All levels: 271828
All levels and monsters: 141421
All special abilities: 011235
Invincible to military attacks: 986960
Demo mode with two random monsters: 082864
Instant demo mode with two random monsters: 874098
View opening sequence: 667300
View ending sequence: 667301
View credits: 667302
Display version number: 314159
Disable all cheats: 000000