Level 26: The Abandoned Mines 
When you start the level, there's a secret door right behind you (#1) which leads to a big room 
with lots of bad guys and items. The teleporter on the acid floor in the west area of the room 
takes you to a platform where you can jump and get the plasma gun. When you start the level, 
there's a secret door right to the east of you (#2). Hit the switch to lower the room first, 
then hit space on the south part of the east wall. This door leads to a network of tunnels that 
take you to many of the acid-floored rooms and monster ledges that you see from your starting
 point. The blue keycard has a secret door behind it (#3) revealing a switch that lowers two 
lifts, revealing two areas to the north and south of the acid pool to your east, which contain 
some cacodemons. The south area contains a trigger line that raises the floor near the east door,
 and a switch which opens that door. The north area contains blue armor, and a secret door on 
the east wall (#4) leading to some health potions and a supercharger. 

Level 27: Monster Condo 
When you start the level, the first thing you should do is go through the door in the northwest 
corner of the room (across the acid pool) and go west to the lit area with the ammo (#1). You 
have 30 seconds after the level begins before this area is closed off. East of the acid pool with
 the rocket launcher in the center is a west-east dividing wall. On the east wall, north of the 
dividing wall, is a lit area in the corner. When you walk into this lit area, a door opens to the
 east revealing an area (#2) containing partial invisibility, light amplification, energy cells,
 and lots of monsters. South of the dividing wall is another lit area along the east wall. When 
you walk into it, a door opens to the east revealing an area (#3) containing some ammo, health, 
shotgun shells, and arachnotrons. In the northeast room with the impaled bodies everywhere, there
 are two switches to the north. The east one lowers a lift to the southwest, which leads to a 
long corridor of health potions (#4) and a teleporter (#5) which takes you up to the ledge in 
the northeast corner of the room, where there's health, armor, ammo, and a BFG. I don't think
 it's possible to get #5, since the teleporter's sector is secret, and you can never enter that 
sector, because there's the teleport line is in the way. Also note that you can't get out of the
 ledge in the northeast corner unless the door to it has opened up already. There are two long 
north-south skylit rooms with large teleporters in the center and pictures of a demon on the 
north wall. Each one has a switch on the east wall, which you can use to open up the west wall 
in both rooms, revealing some mancubi, a beserk sphere, a supercharger, and a computer map in
 each room (#6) in the north one, (#7) in the south one. This is the room with the hell knight,
 just southwest of the yellow key, which has the switch that lets you out of the room after you
 get the yellow key. 

Level 28: The Spirit World 
In the south part of the map is a room with five columns and a throne. Walk up to the throne to 
lower the middle column, and then go stand on it (#1). (You need to do this in order to complete
 the level, so I don't see why it's a secret.) When the column raises up again, you will see two
 switches to the north and south. The south one reveals the yellow key. The north one reveals a 
passage beyond the yellow key, as well as four hell knights, an arch-vile, and (fortunately) an
 invulnerability sphere, some rockets, and a teleporter which takes you to the ledge on the east 
side of the room (#2), where you can get another invulnerability sphere and a megasphere. In the
 southwest part of the map is a hallway which leads west and then north. At the bend in the 
hallway is a small niche to the south containing an arachnotron, who will come out and attack 
you. To enter the niche (#3), just walk through the south wall. There is also another niche 
farther north containing some chaingunners and troopers, as well as ammo and health (#4). Just 
walk through the east wall. In the northeast part of the map is a room with five columns. As 
soon as you enter, some hidden doors will open up and lots of monsters will come out. As soon as
 you kill all the monsters, look on the automap, and you will see three secret doors in the 
niches where the monsters were. The southwest niche has a secret door leading east to a 
supercharger (#5). The southeast niche has a secret door leading north to four backpacks, a 
medikit, and a revenant (#6). The northeast niche has a secret door leading east to a mancubus 
and a beserker (#7). 

Level 29: The Living End 
No official secrets. 
Northwest of your starting point is a path which leads west to a teleporter. As you approach the
 teleporter, some windows open to the north exposing some chaingunners, who promptly attack you.
 If you walk north off the ledge and then walk west around the structure the chaingunners are in,
 you'll see a small opening leading to a teleporter. This takes you to the ledge the chaingunners
 are on; there's a plasma gun there. 

Level 30: Icon of Sin 
The only secret here is how to complete the level. At the south end of the final room is a switch
 which raises the platform in the lava to the north (with the box of rockets). Hit the switch and
 wait for the platform to go up. Then go to the platform, hit space to lower it, and get on. Just
 before it gets to the top, shoot a rocket into the exposed brain of the demon. Then jump down 
and lower the platform again. This takes about three rockets. 

Level 31: Wolfenstein 
No official secrets. However, the room just before the exit door has a secret door in the 
southwest corner, which leads to a small niche with a medikit. The west wall of this niche has 
another secret door which leads to a second exit door; super-secret level (level 32). Just south
 and a bit east of the second exit door is a secret door which reveals a supercharger. There are
 three other secrets which are easy to find on the automap, but which are included here anyway
 for completeness. 
A. There's a north-south corridor with west and east alcoves. The east one contains a body 
impaled on a stake. The west one contains a secret door leading to a rocket launcher. 
B. There's a big square room with a picture of Hitler on the north wall, with trees on either 
side. Hit space on the picture to reveal a secret door to room with a plasma gun and some ammo.
C. Here's one that wasn't in the original game. There's an area in the southeast with lots of 
demons in a "kennel" (there were dogs there in the original game). There are alcoves on either
 side. The last one on the right is marked by some skulls on a stake. That alcove has a secret 
door leading to a BFG, a megasphere, a super shotgun, and lots of backpacks. 

Level 32: Grosse 
No official secrets. However, in the middle of the east wall (the middle Swastika) is a hidden 
door which reveals an invulnerability sphere. The west wall has a hidden door in the same place,
 leading to a bunch of medikits, and a plasma gun with lots of energy cells. Also, the two 
northernmost and two southernmost pillars contain various weapons, and a megasphere; walk up to
 them and hit the space bar.