Enter in the following cheat codes during gameplay for the desired effect.

In map mode, hit IDDT *TWICE* to see all objects, you, monsters, items, and all, in the form of little triangles.

Hold the fire button down on any gun for a little while and the main character's face will get very intense and angry looking.

On the final level, turn on clip mode (type ''idclip'' during game) and walk towards the final bosses exposed brain. You should enter a small hallway behind the hole to his brain, and see that there is actually John Romero's head on a stick. You deafeat the final boss not by destroying the monsters brain, but by shooting the decapitated head of John Romero

Once again, this level is not in the German version of Doom II

To get to the second secret level from the first, go to the last room (with the 12 SS guards and the elevator) and immediately turn to your right, and open the wall back to a secret area with health. Grab the heath, turn around and press that wall to get to another secret are with a elevator to the second secret level, another knock off of Wolfenstien 3D, only this time the level is a remake of Grosse, the first boss.

On level 15: Industrial Zone, activate the switch on the pillar in the large lava trench. Next grab the invulnerability in the now open area at the east end of the large lava trench. Finally enter the southwestern most building on the map and take the small lava river south until you find the newly opened branch with the secret exit teleporter. The branch will not be open if you skip the invulnerability!

If a monster is ever crushed by a door at the exact time an Arch-Vile resurrects it, the monster will become an invincible ''ghost''. This is a very annoying glitch because the ''ghost'' can attack you and float through walls! The only way to stop it is to avoid it at all costs.

If you've managed to encounter a ghost monster and can't kill it, here's how: whip out your rocket launcher or BFG and shoot a nearby wall when the ghost monster is near it. It will be harmed or killed due to splash damage.