The original Doom contains a level known colloquially as the Developer Room — accessible only via console commands or significant sequence breaking on MAP01 — that was never intended for players to see. It contains every weapon, every monster, and every power-up in the game arranged in flat rows, lit evenly, with no threat. It was the developers’ testing space.
Reaching it legitimately is considered impossible in the context of normal play because it requires traversing a wall that closes behind you. Some speedrunners have used rocket launcher blast-jumping to reach ledges that provide line-of-sight to the room’s contents. No legitimate path into the room from the main level geometry exists.
Doom’s secret levels — E1M9, E2M9, E3M9 — are the more intended form of its hidden content. Each is accessible from a specific level through a false wall or hidden switch, and each is designed as a complete level rather than a testing environment. E1M9 is the most polished, E3M9 the most difficult.
John Romero’s level design philosophy for Doom was that secrets should be findable by players paying attention to wall textures and lighting — a secret door should be slightly differently lit or textured from the surrounding wall. This principle, established in 1993, became the design language for hidden content in first-person games for the following decade.
The environmental storytelling in this game is on another level. Thanks for documenting it so clearly.
The detail work the devs put into areas most players never visit is what separates great games from good ones.