- This level occupies the map slot E2M9. For other maps which occupy this slot, see Category:E2M9.
- This level occupies the map slot MAP55. For other maps which occupy this slot, see Category:MAP55.
- This level occupies the map slot MAP23. For other maps which occupy this slot, see Category:MAP23.
- This level occupies the map slot MAP16. For other maps which occupy this slot, see Category:MAP16.
- This level occupies the map slot E2M6. For other maps which occupy this slot, see Category:E2M6.
E2M9: Fortress of Mystery (MAP55 in PSX/Saturn, MAP23 in Jaguar/3DO/GBA, MAP16 in32X, E2M6 in SNES) is the secret map of The Shores of Hell in Doom, which can be accessed from E2M5: Command Center. It was designed by Sandy Petersen and uses the music track "Untitled (Mouth For War)". The par time is 2:50. This map is accessed in the Jaguar and 3DO versions (in which it is called "Dis") through the regular exit from E3M7: Limbo, in the SNES version by finding the secret exit in E2M3: Refinery, and in the 32X, Saturn, and PlayStation versions by finding the secret exit in E2M7: Spawning Vats.
Like many secret maps, this is something of a concept level, featuring a room full of Barons of Hell and a room full of Cacodemons and not much else. The objective is clear the room by using unorthodox methods. On the other hand, if you survive, you can leave carrying every possible weapon (excluding the BFG9000 which does not appear until the next episode).
Completing this level takes the player to E2M6: Halls of the Damned. The Saturn and PlayStations versions take the player to MAP16: Hell Gate, the SNES version takes the player to E2M4: Deimos Lab, and the Jaguar, 32X, and 3DO versions end the game.
- Letters in italics refer to marked spots on the map. Sector numbers in boldface are secrets which count toward the end-of-level tally.
When the level starts you will appear in the center of the "star"-shaped room (A), facing north, and immediately be approached by four Barons located in the northeast, northwest, southeast and southwest "arms". Go through the door in the eastern "arm". The next area (B) holds ten cacodemons. Head to the eastern end of this "+"-shaped room. There are two lion-faced reliefs on the wall (D). Open the wall between them to reveal the final area. Press the skull switch to your immediate left to get the blue key (E), then open the blue door to get the red key (F). Open the red door to get the yellow key (G), and lastly open the yellow door to reveal the exit teleporter (H).
Other points of interest
At the start point, there is a blue armor directly behind you.
The "star"-shaped room is easiest to complete if you have equipment leftover from previous levels: find a safe spot and take the Barons out with your most powerful weapon. Once they've been taken care of, look in each of the "arms" to find (clockwise from north) a plasma gun, rocket launcher, shotgun, chainsaw, computer area map, backpack and a chaingun.
A much safer option is to turn the aggression of the Barons and Cacodemons against them. If you immediately start running in circles around the barons, you will shepherd them into a closely spaced group. Now run into the "+"-shaped room and do the same with the Cacodemons; do not fire as this gives them time to zero in on you. Then run back to the Baron room, stand between the two groups until they have damaged each other (accepting a blast or three yourself), and then run into one of the corners. At this point, you can expect to be mostly left alone as the Cacodemons and Barons will have hit each other and start a huge confrontation sequence. You can even sit back and watch the spectacle, then finish off whatever creatures remained, though these may be badly wounded by now. You'll probably regain any health lost and more by cleaning out the "+" room, and won't have lost much ammo (if any). The fact that there are preexisting corpses of each monster type in the other monster's room suggests that this was the designer's intended strategy.
- The exit room counts as a secret (sector 5).
- The yellow key can be grabbed through the red door without opening it.
- Similarly, the red key can be grabbed through the blue door, or through the wall to the west.
- One energy cell (Thing 37) has no flags set for any of the difficulty classes, so it does not actually appear in the level.
- Sidedefs 135, 138, and 158 are missing their upper textures. This has no actual effect on rendering because each key chamber cannot be entered until the back of the door has risen beyond the player's field of view (even with the no-clipping cheat, the resulting anomalies are fairly narrow in the latter two cases; the first case creates a HOM over the entire door).
- No monsters, 100% secrets walkthrough (file info)
- 100% kills, items, and secrets on ITYTD (file info)
- 100% kills, items, and secrets on UV (file info)
Areas / screenshots
Routes and tricks
In a straight speedrun, the yellow key grab is essential for a world-class time. Marijo "Sedlo" Sedlic credits Thomas "The Godfather" Zajic with the original discovery.  If you choose to open all the doors, you may need the soul sphere or the blue armor first, as you will expose yourself to many more projectile attacks thereby. You can also shave a few seconds  by approaching each key obliquely, allowing you to rebound off the door jamb.
A fast time in NM speed may actually be easier than in UV speed, because the two fast cacodemons in the rear shoot at you a lot instead of moving into the center of the room.   Given this, and given the completely straight path between the start point and the secret door, 0:06 is arguably an optimal time.
- Pick up the backpack, rocket launcher, plasma gun, shotgun, and blue armor.  Because the weapons are not all adjacent, the barons should cluster near the center of the room, so they all make it through the door in the next step.
- Get the soul sphere and open the secret room.
- Shoot cacodemons with the shotgun from there, using the reload times to activate doors and pick up keys. 
- Once the exit chamber is open, cross to the other side of the cacodemon group and continue firing until a predetermined number of shells remain. (The extra health and armor are most significant here, because you can't see the baron attacks coming.)
- Run back to the west side of the "+" room, use up your rockets (preferentially on barons), and finish everyone off with cells.
Although the chaingun has a higher damage rate than the shotgun, its ammo is more limited ; the bullet boxes are also out of the way of the main route, whereas the cacodemons seem to enter the crossfire sooner if one approaches the secret door promptly.  Luck plays a role in achieving a world-class time; one must hope that the barons stay close enough together to take a lot of blast damage , and that the infighting has sufficient overall effect that you don't run out of ammo at the end. 
In the UV -fast style, the opening gambit from UV max does not work well due to the barons' increased attack frequency, nor is it nearly so safe to stand between the barons and cacodemons for an extended time. Instead, pick up the shotgun and enter the cacodemon room, shooting a few monsters while collecting ammo, and proceed to the secret area. Go back for the other weapons only after opening all the doors.  This allows enough time for infighting to begin, which of course damages the monsters more quickly once it has. Also, while in the exit area, sidestep the incoming projectiles more fastidiously than at normal speed. 
No known route optimizations exist for UV -respawn.
Early Tyson submissions allowed some infighting to proceed and then used the chainsaw exclusively, the total time determined by how long one waited to start hitting barons from behind. It is faster, however, to go straight to the exit and pick up the keys in several sorties, circlestrafing with the pistol in between.  The current record holder collects all the keys at once, then herds the monsters into a small area by staying near the western door ; this is more dangerous, but faster, since it increases the frequency of monster-monster targeting. An important footwork consideration while chainsawing each cacodemon is to use it as a shield against incoming fire.
UV pacifist is indistinguishable from UV speed on this map, as there is no good reason to attack such large monsters with a pistol.
The Compet-N records for the map are:
|UV speed||00:06||Marijo "Sedlo" Sedlic||2000-05-14||e2m9-006.zip|
|NM speed||00:06||Marijo "Sedlo" Sedlic||2002-11-11||n2m9-006.zip|
|UV max||00:49||Drew "stx-Vile" DeVore||2003-01-21||e2m9-049.zip|
|UV -fast||00:52||Adam Hegyi||1999-08-09||f2m9-052.zip|
|UV -respawn||00:52||Adam Hegyi||1999-05-26||r2m9-052.zip|
|UV Tyson||01:20||Anders Johnsen||1999-04-05||t2m9-120.zip|
|UV pacifist||00:06||Marijo "Sedlo" Sedlic||2000-05-14||e2m9-006.zip|
- This level is impossible to complete without 100% secrets (unless the noclip bug occurs), so a separate category from NM speed would be redundant.
This level contains the following numbers of things per skill level:
|Monsters||ITYTD and HNTR||HMP||UV and NM|
|Barons of Hell||4||4||4|
|Powerups||ITYTD and HNTR||HMP||UV and NM|
|Weapons||ITYTD and HNTR||HMP||UV and NM|
|Ammunition||ITYTD and HNTR||HMP||UV and NM|
|Keys||ITYTD and HNTR||HMP||UV and NM|
Inspiration and development
- Prior to entry of the level, the fortress does not appear on the intermission screen. After the E2M5 statistics have been cleared, the fortress fades in on the map. Once E2M9 is completed, the fortress disappears again, but the blood splat (which marks that the level was completed) remains in place.
- In the and PlayStation and Saturn ports, this map is accessible from E2M7: Spawning Vats instead of Command Center.
- The whole level seems to occur underground, and the starting area looks not unlike a coal mine. While the other Deimos facilities are slowly being subverted by the forces of Hell and feature a mixed techbase/Hell theme, the fortress features virtually no use of the techbase textures which suggests that this entire "base" is not man-made or has been completely converted by Hell already.
- This level is used as the final level for the 32x, Jaguar, GBA, and 3DO versions of Doom.
- The music for this level is inspired by the track "Mouth for War" by Pantera.
- Level designer Petersen considers this map his best Doom level design, calling it "a little gem of creature and unit balancing".
- This is the only DOOM secret level to not use the song, "Hiding the Secrets".