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 source port Boom yet added exits triggered by impact weapons, including secret exits, which are consequently available in any Boom-compatible port.

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.

Of custom levels, MOUNTAIN.WAD and DARKHELL.WAD have examples of the behavior.

In the former, the player skips a regular level upon finishing the secret one:

In the latter, the player must play the same level over again:

As a corollary, activating a secret exit within a secret level resets the level (following the usual intermission screen). EP.WAD 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.