Secret level

A secret level is an optional level accessed from a generally concealed alternate exit. Since the secret level uses this second, less accessible exit, the game or episode it forms part of can be completed from start to finish without progressing through the level. A super secret level is a secret level accessed from a previous secret level, as seen in Doom II.

While secret exits could only be set up as switches in Doom, walkover lines and switches were both supported in Doom II and Ultimate Doom. The Boom source port also added exits triggered by impact weapons, including secret exits, which are consequently available in any Boom-compatible port.

Games and ports with some manner of MAPINFO support or equivalent feature allow to modify the hardcoded behavior detailed in this article. Level scripting, such as ACS or FraggleScript, can also be used to reach secret levels by other means than using a secret exit line type.

Secret levels in Doom
Doom has four secret levels: E1M9, E2M9, E3M9, and E4M9. They are accessed from E1M3, E2M5, E3M6, and E4M2, respectively.

The actions of the secret level linedef types have been hard-coded into the engine. If the player activates a secret exit elsewhere than in one of the four aforementioned levels, the player will go to the secret level as usual. However, upon completing it, the player will be taken to the level immediately following the one they were meant to access it from. Depending on the specific episode and level this occurs on, the player may end up either skipping several maps, repeating the level they just left, or being sent back to an earlier level.

For example, reaches E3M9 from E3M5, and continues on to E3M7, skipping E3M6 entirely. Inversely, reaches E2M9 from E2M6, and loops back to it when exiting the secret level. As a corollary, activating a secret exit within a secret level resets the level (following the usual intermission screen). contains an example of this.

Secret levels in Doom II
Doom II has two secret levels: MAP31 and MAP32. The first is accessed from MAP15, and the second is a super secret level accessed from MAP31. Both TNT: Evilution and The Plutonia Experiment, being originally designed as Doom II megawads, follow the same pattern.

If the player uses a secret exit outside of MAP15 or MAP31, the next level will be the one that was last entered via an intermission screen. If such a level is not available, the player will be warped to MAP01: this effect is created by the secret level information not having been initialized. Some source ports change this behavior to something more consistent: for instance in ZDoom the Doom II secret exits outside of MAP15 and MAP31 merely lead to the same level as the normal exit.

Secret levels in Heretic
Heretic uses the same system as Doom. There are five secret levels, one per episode: E1M9 (reached from E1M6), E2M9 (from E2M4), E3M9 (E3M4), E4M9 (E4M4), and E5M9 (E5M3).

Heretic also features three hidden levels which are not reachable through normal gameplay and do not have automap names: E6M1, E6M2, and E6M3.

Secret levels in Hexen
Hexen, thanks to ACS, hubs, and MAPINFO, offers a lot more flexibility in its secret levels than the various Doom games and Heretic. Each hub features a secret level: MAP06, MAP11, MAP31, MAP26, and MAP39. They are accessed from MAP02, MAP13, MAP27, MAP21, and MAP35 respectively, although in many case it is necessary to perform actions in other maps as well before the access to the secret level is opened.

The expansion pack features secret levels in each of the single-player hubs: MAP47, MAP53, and MAP59. They are reached, in order, from MAP41, MAP49, and MAP53.

Secret levels in Strife
While Strife does not feature any officially secret levels, and the concept of a secret exit switch does not exist in its variant of the Doom engine, some of the maps in the game are unofficially considered to be secrets due to limited access, a lack of explanation given to them in the game, or their remote and optional nature which requires exploration by the player. Maps usually classified in this manner include:
 * MAP22: Factory: Forge. An out-of-the-way area of the Factory which contains moderate resistance and a number of power-ups and inventory items to collect, but does not serve to advance the plot in any way and is not required to progress. Unused NPC characters located here indicate that the level may have originally played a larger role in the game.
 * MAP30: Abandoned Front Base. After defeating the Programmer, the Front Base is replaced by this map, and many goods can be found which were left behind by the Front.
 * MAP31: Training Facility. It is reached from MAP07: Castle, and becomes inaccessible once the Programmer has been defeated. Though not required, the player can earn stamina implant and accuracy stat upgrades by completing its obstacle course.

In Strife: Veteran Edition, a "super secret" level is added to the game as part of its talisman quest. Accessing the map requires that the player find a blue chalice for Timothy and then speak to the Master Smithy in the Forge.
 * MAP35: Factory: Production