Doom shareware

Doom cheats are typed straight into the running game — there is no console and no menu to open. You simply type the code on the keyboard while running through a level, and a confirmation message appears at the top of the screen after the last letter. If you are playing in the browser, click inside the game window first so the keyboard talks to the game and not to the page. This is a first-generation Doom game, so noclip is IDSPISPOPD and IDCLEV takes an episode plus a map — IDCLEV41 sends you to episode four, map one. Shareware Doom contains only the first episode, yet IDKFA still hands you the full arsenal, BFG 9000 included, right in the opening level.

Doom is one of the rare games whose cheats became common knowledge — plenty of people who never played it still know IDDQD. Doom cheats are not hidden in a menu, they are simply key sequences — which is why you can trigger one by accident. They are also worth saving for a second playthrough: IDDQD turns Doom into a shooting gallery and switches off the rule the whole game is built on, namely managing ammo and health.