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.

In Doom, secret exits could only be set up as switches. In Doom II and Ultimate Doom, secret exits could be triggered by walkover lines as well. The source port Boom 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.

If the player activates a secret exit on a level they are not meant to reach a secret level from, they will go to the secret level, as usual. However, upon completing the secret level, the player will be taken to the level immediately following the level 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. One example where this behavior occurs is MOUNTAIN.WAD, where the secret exit is placed on E3M5, which results in the player skipping E3M6 upon finishing E3M9, instead going directly to E3M7. Another example can be seen in DARKHELL.WAD, where the secret level is accessed from E2M6; upon completing E2M9, the player must play E2M6 over again.

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 based on Doom II, follow the same pattern.

If the player uses a secret exit on a map they are not supposed to reach a secret level from, they will end up repeating the level over again. If the level was reached by using IDCLEV or the -warp command line parameter, they will be warped to MAP01.