View network opponent location:
Hold [Alt] and type “iddtiddtiddtiddt” at the map screen.
On level 30, enable the idclip code. Walk into the room with the Demons, Cacodemons, Imps, Lost Souls, and lava. Fall in the lava and keep walking. Finally, watch the marines mugshot and enable the iddqd code.
The Gateway To Hell secret room:
Enable the idclip code at the start of the “Gateway To Hell” level to walk through walls. Also enable the iddqd code for immortality. Walk forward and turn left into the wall opposite the transporter until you are in the main room. Then, turn right and walk forward into a small green room where some enemies are being transported and crushed. Keep looking inside the walls until you find another huge room with weapons and a few of Berserker Packs. Sometimes there is an unkillable Spectre in there.
How to defeat the final Boss:
In the last level, find the wall with a giant demon skull that shoots blocks that turn into demons. Enable the idclip code and walk through it until you see a man’s head (John Romero) on a pike. Shoot or chop it with a chainsaw. You will then have completed the game.
Former Soldiers: They resemble your old friends, but now they just fire pistols at you. They drop clips for the chaingun and pistol when killed.
Former Sergeants: Meaner and tougher. They carry shotguns and do more damage then soldiers. They drop shotguns when killed.
Imp: This creature throws fireballs and takes more bullets to kill.
Demon: Appear in most areas and can rip your head off if you are not careful. If you see more of them, use the chaingun.
Cacodemon: These creatures are ugly and have one big mouth. If they see you, they will fire lightning balls from their mouth. Use your rockets.
Mancubus: You have to use more then a chaingun to kill them. Use ammo with caution.
Arachnotron: Fires plasma energy at you. Use your rifle.
Spider Demon: The ultimate enemy. Has a super chaingun mounted on her body. Use your plasma rifle or rocket launcher.
On the setup screen, set the difficulty to ultra-violence, fast monsters on , and either versions on (TnT or Plutonia). Play, then press [Esc], select the options, then “End game”. Notice that it is basically the same, but then it goes wrong.