Level 11: 'O' of Destruction! 
On the east side of the map is a small room containing a chaingun with green torches on either 
side; you get to it via a lift which lowers when you come down a set of stairs. On the south side
 of this room is a secret door, which opens onto a hallway (#1) which leads to a ledge where you
 can get partial invisibility. Also, note that there is a supercharger west of the set of stairs 
mentioned above, on the left (south) side of the door. Just walk through the wall to the right of
 the green torch. On the east side of the outer part of the circle (north of the walkway leading
 east, south of the outcropping with the supercharger on it) is a hidden door in the slime. It 
opens when you cross the trigger that reveals the arch-vile. The door leads to some boxes of 
rockets (#2) and a teleporter which takes you to some energy packs in the center of the circle. 
In the northwest corner of the map is a series of bars which open automatically when you approach
. Just north of the bars is a secret door (#3) leading west to the hell knight and 
invulnerability. 

Level 12: The Factory 
In the northwest corner of the big main building is a room with jagged walls (#1); it contains a 
BFG9000 and lots of chaingunners. To open the door leading there, you must hit the switch in the
 southeast corner of the room with the blue door facing northwest. Note that there is a radiation
 suit in the southwest corner of that room. The room with the blue door facing northeast (with 
the platforms that lower as you reach them) contains a teleporter to a ledge facing northeast 
outside the building. From there, you can jump down to the southwest and get three boxes of ammo 
(#2). Hopefully you already killed the mancubus. This room also contains a switch which helps you
 get secret #4. In the southwest corner of the map is a small building with some crates. High on 
the east wall are a chaingun and some ammo (#3); you can get to them by stepping from crate to 
crate until you get to the lift (the switch), and then use the lift to get up to ledge they're on
. In the southwest corner of the room is a teleporter which takes you to the square structure in
 the center of the big main building. If you hit the switch mentioned in #2 already, then the 
center of this structure will be raised up so that you can just grab the supercharger (#4). If 
you haven't, then you'll be surrounded by four imps scratching away at you. Kill them and hit the
 switch, which will raise the floor up so you can get the supercharger. 

Level 13: Downtown 
There is a building in the southeast with a switch next to a lift. Hit the switch and take the 
lift up. There are two doors; the right door won't open. Take the left one and jump across the 
slime pit to the other side (#1); there's a computer map and a chaingun. In the southwest corner 
of the building in the southwest corner of the map is a switch which opens a door leading outside. 
There's a medikit and some ammo out there (#2). Just to the east of the switch is a stack of 
crates which you can lower by hitting the space bar, revealing another switch. After you hit the
 switch, you can walk around the stack of crates to your north to find a set of stairs leading up
 to a teleporter. The teleporter takes you to a ledge just west of the starting point (#3). From
 this ledge, you can run north into the building with the supercharger. The building in the 
southwest corner of the map has a pit in the northwest corner, which leads to a lift. The lift 
takes you up into another building. Just across from the lift (to the east) is a switch; hitting
 the switch opens a door revealing some shotgun shells (#4). The building with the revenants 
guarding it (the one with the blue door at the entrance) contains a crate with a switch on the 
north side. You can hit the switch to lower the northeast platform, so you can get the chainsaw 
(#5). The structure with the red keycard on top of it has a hidden door in the middle of the 
north wall, which can be reached from the ground. The door opens to reveal some blue armor (#6).
 In the northeast corner of the map is a teleporter which takes you to the ledge of a building 
with some imps. Jump over to the ledge of the building just to the west. There's a door there 
which leads to a teleporter, which takes you to the interior of a building farther north, right 
between two cacodemons (#7). There's a lot of ammo in there too. Hit space on the inner wall to 
get out. The building just southeast of the crusher with the plasma gun has a door in the south 
wall with a stairway leading up to a blue door. Once you have the blue key, you can enter out and
 get some ammo and rockets (#8). 

Level 14: The Inmost Dens 
No official secrets. One way to get the supercharger is to go to the building east of the 
starting building via the bridge. Enter the building through the south door. Run out one of the
 windows facing west into the moat, south of the bridge. Lower the lift in the southwest corner,
 then go through, and go east, kill the arachnotron, and get the supercharger. 

Level 15: Industrial Zone 
Get the red card key. (See below for help on how to do this.) Then jump down to the ground, just 
to the east of the platform the red key is on. There is a beserk box there. Hit the space bar on 
the crate just to your east to lower it, and get the stimpack (#1). The red card key is not a 
secret, but a lot of people have trouble finding it, so here are some directions on how to get 
it. Go to the building with the staircase; the one with the lava pit around it. It's northwest 
of the diamond-shaped building. Walk up the stairs to the top. Stand in the southeast corner of 
the top floor and run west into the teleporter on the other structure you see in front of you. 
The teleporter will take you to the ledge of another building. Walk straight ahead (east) to the 
end of the ledge; you'll see another ledge, on the red key building to your east. Kill any 
monsters you may see on the ledge. Then, jump over to that ledge (don't get too much of a running
 start, or you'll hit the wall above it). Hit the lion switch, then jump down into the building 
and take the teleporter in the southeast corner. You'll appear in another teleporter. Walk 
forward a bit and then step back into the teleporter, then take the red key you see in front of 
you. An easier way to get this (but probably not the "official" way) is to go up to the building 
with the red key in it, just southeast of your starting point. Walk to the southwest corner; in
 the middle of the south side of the building is a corner, where a small bit of wall juts out. 
Stand in this corner facing northeast and hit space. (You can also hit this switch from inside 
the building.) Then enter the building through the lift in the northwest corner, and take the
 teleporter in the southeast corner. Walk forward, then backward, then take the red key. Take 
the teleporter in the southeast corner of the red card key building. Walk out onto the ledge, 
then go to the north end and drop into the opening to the west, falling down to the main floor 
of the building. Then walk out the west exit. Go back and take the teleporter in the red card 
key building again, and walk north and west again; this time, the floor has risen up to the level
 of the ledge on the north side of the building. From here you can jump south onto the platform 
with the ammo and chaingun (#2). Go to the top of the building northeast of the diamond-shaped 
building. Just north of the teleporter is a secret door on the east wall (#3). Go through down 
onto the acid pool; to the north is a backpack and some rockets. Go to the top of the 
diamond-shaped building on the north shore of the acid lake; walk to the south ledge of the 
building. Jump south down onto the small island with the radiation suit and energy cells. (Try 
walking instead of running.) Hit the switch, then jump off and go northeast to the east side of
 the lake. Go through the door you find there; through it, you'll find a platform with an 
invulnerability sphere (#4). In the southeast corner of the map is a building with a spiral 
staircase. Go up and around the staircase. As you turn to go south, a door will open to the east.
 Run south across the lift, then east, then run north, jumping across the lower hallway; there 
you'll find an open door (#5) revealing a megasphere (#6). Now go up the stairway again. As you 
go south over the lift, a door will open up on the north wall, behind you. Go through the door 
into an area with energy cells and lost souls (#7). Go east into a small opening with a switch. 
Hitting the switch will cause a teleporter to appear in the hallway to the west. Enter the 
teleporter; it will take you to a platform in the southwest corner of the building with a 
chainsaw (#8). Go the building in the southwest corner of the map. At the south end, you will
 find some stairs going east (unless you haven't hit the switch to raise them). At the top of 
the stairs is a secret door to the east, revealing a small area (#9) with a teleporter to the 
top of the square spiral staircase building northwest of the diamond-shaped building. I was not 
able to get credit for secret #9, even if I walked into the teleporter; so the most I could get 
for this level is 90% secrets. Instead of taking the teleporter, you could go to the top of the 
building to the ledge looking north. Hit the switch on the west wall, then jump down to the 
ground and go back through the front door of the building. Go into the acid river and follow it
 north to the lift, which takes you up to another ledge where you'll find a plasma gun and ammo 
(#10). Now go back to the south end of the building and go south through the acid river tunnel.
 If you got secret #4, there will be a secret door leading south in the tunnel (#11) leading to a
 teleporter to the secret level (level 31). 

Level 16: Suburbs 
There are two buildings in the center of the map. You start in the north one; enter the south one
. In the south part of the building is a little platform with some shotgun shells on it. Hit 
space to lower the platform, then get on it. When you get to the top, shoot at the niche in the 
wall to your east. A megasphere will be revealed in the southeast corner (#1). (It's easy not to
 get credit for this secret, even if you do get the megasphere. Walk slowly into and out of that
 corner, making sure you drop into the depression there.) Then turn northwest and shoot at the 
eyes in the wall. You may need to do this while the platform is rising for it to work, or you 
could try using a shotgun. If you hit it, a door will open just behind the wall to your north, 
containing some ammo (#2). On the east side of the map is a building with doors on each wall, 
but you can only go through the door in the east wall. Open the east door and enter the building
 (#3). Hit space on the northwest platform to lower it, then get on. When it rises back up, walk 
south into the acid pool inside the southwest platform, then hit the switch. The platform will 
lower, revealing a small room to the northwest. Inside the room is a BFG9000, some energy packs,
 and a medikit (#4).