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
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, MOUNTAIN.WAD reaches E3M9 from E3M5, and continues on to E3M7, skipping E3M6 entirely. Inversely, DARKHELL.WAD 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). 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: 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
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.