The first of the Cheat Codes can be found near the end of Hell on Earth. Walk through the corridor in the image below, and you'll see a yellow question mark hidden behind a gate above you. Continue ...

It just wouldn't be a Doom game without a haul of cheat codes allowing you to do anything from granting the slayer constant power ups, to instantly unlocking all Rune or Praetor Suit upgrades. Heck, ...