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The Evolution of Doom's Level Structure

[Doom alpha screenshot] [Doom alpha screenshot] [Doom screenshot]

During its fifteen-month gestation in development, Doom changed dramatically. Originally anticipated as a slow-paced role-playing game, it ultimately evolved into a fast-paced action game as elements were stripped and refined.

At various points of development, linear levels, non-linear levels from a central hub (à la Commander Keen or Hexen), and even the idea of a massive single level were all explored. In the "Doom Bible" design document dated 28 November 1992, Tom Hall tabled the ideas for six distinct episodes, paralleling the structure of Wolfenstein 3D:

The Doom Bible's outline for the first episode also contained descriptions of fifteen "buildings" expected to serve as levels. It doesn't take higher math to realize that 15 buildings times 6 episodes equals 90 levels...and this level structure would have surely destroyed Id by setting its designers up for an insane amount of work.

Unsurprisingly, cuts followed. The six episodes were soon distilled into three following a "Techbase" → "Hell" → "Corrupted Techbase" progression, with the last two switched in the final months of development. Likewise, the number of levels per episode was progressively reduced from 15 to 13, then to 9. The version of Doom released in December 1993 ultimately contained 27 levels.

In spite of the move away from its structure, the spectre of the Doom Bible loomed large as development progressed in early 1993...and nearly all of the levels that appeared in the first alpha prototypes were direct implementations of techbase locations from the design document, in the same order:

Building Doom Bible Name Doom Bible Description Implementation
1 Hangar 2 "The secondary hangar fell into disuse after the glamour faded from the Tei Tenga bases. It now stores used up equipment, junk, and the room where the soldiers play cards. Sunk into wall of canyon. You can see out a window on the second floor. The hangar has become grungy since it has fallen into disuse." E1M1,
Doom 0.3 Alpha.
2 Supply Depot 2 "Still used as a storage depot, but not as active as the main supply depot. Holds lots of stuff." E1M2,
Doom 0.3 Alpha.
3 Waste Processing Facility "Processes all the waste for the whole facility, splits it into water, nutrients for the lab's gardens, solid fuel, and a bit of unusable waste which is either jettisoned from the dome or used by the cook." E1M3,
Doom 0.3 Alpha.
4 Enlisted Quarters "Where all the grunts, including our soldiers, sleep, shower, and shit. Have their names on some lockers, perhaps, or have the lockers respond to them. There are five soldiers for each shower and toilet." E1M6,
Doom 0.5 Alpha.
5 Tower "Communications tower, this contains a huge antenna, comm equipment, and soon. This is usually inhabited by robots (non during the game, or at least game one), and is just jam-packed with dangerous technology (as underneath the efficiency of the UAC design, the nuts and bolts are sucky, from committees and cost-cutting (profit-increasing) measures. Also, only robots are gonna see it, right?"
6 Recreation & Training Center "Where the approved recreational activities take place, and where the soldiers train." E1M7,
Doom 0.4 Alpha.
7 Mess "Where everyone eats. 'Mere are kitchens, and a pipeline to the Waste Processing Plant."
8 Officers' Quarters "The COs stay here. It's a bit nicer than the enlisted place, and a locked door leads to the Ammo Storage."
9 Personal Storage "Lucky is the player who figures out how to get in here. There is just lots of stuff here to shoot demons with. You definitely need the Colonel's Hand, the power must be on, and you must find the side passage off the monorail system by the quarters."
10 Control Center / Power Plant "This is the gateway between the everyday side of the base and the restricted, secret, much-coveted-by-the-player side." E1M8,
Doom 0.3 Alpha;
Doom 0.4 Alpha.
11 Lab "Set up for the study of Fire Dust, the anomaly, the moon, and so on. Also contains some experiments best carried out away from the mainstream." E1M13,
Doom 0.5 Alpha.
12 Observatory "It is an observatory, but full of nasty demons and lots of treasure. Imps got here easily, and have set up their supplies here." E1M10,
Doom 0.4 Alpha,
13 Supply Depot 1 "The main supply depot everything that comes off a ship is stored here, then routed through the underground tunnels to the other buildings." E1M11,
Doom 0.4 Alpha.
14 Anomaly "In the bowels of this structure is the anomaly itself, and the entrance to the hell canyon. You can only reach this structure from the lab's basement." E1M12,
Doom 0.4 Alpha.
15 Main Hangar ("Will probably be cut") "Where ships usually fly in. You can here from Supply Depot One, if you find the access tunnel. There's a lot of neat junk in here."

The Doom Bible origins of levels are fascinating, and explain the design choices evident in certain maps. For example, the Recreation & Training Center was implemented as E1M7 in the April 1993 0.4 alpha, where it contained locker and shower areas. The lockers were ultimately removed during the course of development, and the map was recast as E1M4 (Command Control) in the finished game. Yet even the final version contains a theatre stage area, hinting at the "recreational activities" that were once cast as its purpose.

The level designers of Id Software had a tendency to develop episodes and maps in order, and this was evident during the development process of Doom. Doom 0.3 Alpha contained 4 first-episode maps out of an intended 13; Doom 0.4 contained 10 out of 13, and Doom 0.5 contained a complete first episode plus one map intended for the second. Yet maps were also replaced, dropped, shifted, or shunted around, particularly after Tom Hall left and the number of levels per episode was reduced:

Original Name Creator Doom Bible
Doom 0.3 Alpha
Doom 0.4 Alpha
Doom 0.5 Alpha
Press Release Pre-Beta
Final Version
Final Name
"Techbase" levels originally intended for Episode 1:
Hangar 2 Hall E1M1 E1M1 E1M1 E1M11 E2M7 Spawning Vats
Hall E1M1 (dropped)
Romero E1M1 Hangar
Supply Depot 2 Hall E1M2 E1M2 E1M2 E1M2 E3M2 E2M2 Containment Area
Waste Processing Facility Hall E1M3 E1M3 E1M3 E1M3 E2M3 Refinery
Romero E1M4 E1M4 E1M7 Computer Station
Romero E1M5 E1M9 E1M6 Central Processing
Enlisted Quarters Hall E1M4 E1M6 (Doom II MAP10) Refueling Base
Recreation & Training Center Hall E1M6 E1M7 E1M7 E1M4 Command Control
Power Plant Romero E1M10 E1M8 E1M8 E1M8 E1M2 E1M2 Nuclear Plant
Control Center Hall E1M10 E1M9 E1M5 E3M3 Pandemonium
Observatory? Romero E1M12? E1M10 (dropped)
Romero E1M10 E1M3 Toxin Refinery
Supply Depot 1 Hall E1M13 E1M11 (dropped)
Anomaly Hall E1M14 E1M12 E1M8 Phobos Anomaly
Lab Hall E1M11 E1M13 E2M4 Deimos Lab
"Hell" levels originally intended for Episode 2:
Hall E2M1 E3M7 Limbo
Petersen E2M5 E3M5 Unholy Cathedral

    = First appearance.
    = Position changed.

Doom 0.4 and 0.5 Alpha's Semi-Nonlinear Level Structure

[Doom 0.5 intermission map]

Hints of non-linearity are evident in Doom's pre-release development alphas. In 0.4's E1M9, two corridors branching from the starting area are labelled with wall signs marked "WH1" (Warehouse 1) and "Lab" (Laboratory), signifying the intent for the level to contain portals to both.

By May 1993 exit switches had been implemented, and an intermission map graphic was added showing connected pathways between buildings. (This graphic aligns more closely with the map layouts of 0.4 Alpha than 0.5, suggesting it was drawn at a stage of development between both.) Although no level had more than one exit, several of the exits advanced the player to a connected building as shown on the map, rather than the next sequentially-numbered level:

Abbr. Name Level (0.4) Level (0.5) Connections shown Exits to
"Public" side
Han Hangar 2 E1M1 E1M11 SD E1M7 (Rec)
- E1M1 E1M2 (SD)
SD Supply Depot 2 E1M2 E1M2 Han, Com, Waste, Ref (not reachable)
Waste Waste Processing Facility E1M3 E1M3 Rec, SD, Ref E1M7 (Rec)
Ref Refinery Rec, SD, Waste, Pow, Quart
E1M4 E1M4 (Itself)
Com Communications Tower Contr
E1M5 E1M9 (no exit)
Quart Enlisted Quarters - E1M6 Pow, Ref, Quart E1M2 (SD)
Rec Recreation & Training Center E1M7 E1M7 SD, Waste, Ref E1M3 (Waste)
Pow Power Plant E1M8 E1M8 Contr, Ref, Quart (no exit)
Contr Control Center E1M9 E1M5 Com, Pow, Lab, Ware (Itself)
"Restricted" side
Obs Observatory E1M10? - (None) (not reachable)
Ware Warehouse 1 E1M11 - Contr, Lab, Anom E1M7 (Rec)
Anom Anomaly - E1M12 Lab, Ware (no exit)
Lab Laboratory - E1M13 Contr, Ware, Anom E1M3 (Waste)
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Last update 23 November 2023.