Level 17: Tenements 
Go to the center of the map, then go east until you come to two staircases leading north and 
south. Lower the lift to your south by jumping down in the slime just to the west of it and 
hitting the switch. Just south of the lift is a hidden room with some armor. On the west wall 
of this room is a lift (#1) leading to a chaingun. In the north part of the map is an M-shaped 
structure. Walk north down the middle of the M into the acid lake, then turn around and hit 
space on the wall. The middle of the M will be lowered, and you will see a switch to your south.
 Run south and hit the switch. (Actually, you don't have to lower the walkway to hit the switch; 
you just have to stand in the right place and press the space bar.) A door will be revealed to 
the east; you can jump into it and get some armor (#2) and a BFG9000. On the way to the exit, 
just to the southwest, is a lift. As you go up the lift, you'll notice an opening to the south. 
While the lift is moving, run into the opening (#3) and into a hallway leading east to a 
megasphere. Also, the room with the really annoying stairway north and west of the starting 
point has four teleporters on the floor. The northwest and southeast ones go to the north and 
south ledges where the monsters appear. 

Level 18: The Courtyard 
When you walk around the pillar in the center of the main area, a number of doors will open up, 
including two against the wall west of the center pillar. Of these two doors, the north one leads
 to a BFG9000 and some more monsters (#1). (The south one has a hall of mirrors effect in the 
door tracks.) When you walk counterclockwise around the pillar in the center of the main area, a
 door will open up northeast of the pillar leading to a teleporter, which takes you to the 
platform southwest of the pillar. The platform (#2) contains some rockets, health, and blue armor
. The teleporter will not be there until you hit the switch on the northeast corner of the 
plus-shaped structure just to the south of it (the one containing the cacodemons). In the 
northwest corner of the map, hidden between the structures on the north and west walls, is a
 corner with a small bit of grass. (#3) Walking into the grass teleports you to the northeast 
corner of the map, on a ledge containing a medical kit and a computer map. (Note that the 
building just to the east of the bit of grass has an unclosed sector inside it; you can see out 
the east wall.) In the east area of the map is a hallway ending at a blue wall with a skull 
switch. Hitting the switch opens a hidden door revealing a chaingun (#4). 

Level 19: The Citadel 
In the southwest corner of the main building is a room with a switch in the middle which opens 
doors leading north and south. Hit the switch and go north. At the intersection, go west, and 
north through the door into a room with health, partial invisibility, and lots of imps (#1). 
Then go back to the intersection and go north. At one point, you'll meet a hallway leading off 
to the northwest. Just south of this hallway on the west wall is a secret door. (It's just north
 of the place where it LOOKS like there's a secret door.) This door leads through a short hallway
 (#2) to a lift, which leads to a hall leading northwest (#3) and stops at a window. Jump out the
 window to the northwest. You should be able to reach the steps of the building to the northwest
 (#4). The building has two openings leading inside. Try the south one first; when you finish in 
there, an area will open up to the northeast with lots of troopers and a computer map (#5). The
 north opening leads (eventually) to five teleporters. The middle one leads to the pillar in the
 middle of the castle, which contains a rocket launcher. The other four lead to the four corners
 of the castle. Take the northeast one; in the northeast corner of the castle (#6), you will find
 the BFG9000, some demons, and the blue key. Or, take the southwest one; there you will find a 
chainsaw and the yellow key (#7). An easier way to get to the southwest tower is to go to the 
southwest room of the castle with the doors on the north and south; take the south door, and 
walk down the corridor until you hear a door open, then run back and walk through the open door
 to your left. In the southeast corner of the map is an area at the top of some steps with two
things that look like switches to the north. They're not switches, but there's a hidden door 
between them. Go through the door and up the steps, and hit the switch you see there (on the 
east side of the wall) 3 times. Then jump down to the north onto the platform
 you've raised up, and from there jump onto the platform in the northeast corner of the room. Go
 through the teleporter, and you'll get a supercharger (#8). Go south a bit and then north, and 
you'll be teleported to a platform in the southeast area of the castle. Jump to the platform to 
the northwest (this platform appears when you shoot at the imps in the southwest corner; the room
 that they are in is missing its north wall, BTW) and go through the door, then down the steps to
 get the red skull key and some rockets (#9). 

Level 20: Gotcha! 
The structure just south of your starting point consists of a building with a walkway around it 
to the south entrance. (The walkway may not be there if you have not tried to get to the 
structure; just walk south from the starting point to raise the walkway.) In the southwest part 
of the "moat" (the poison area between the walkway and the building) is a small opening, 
containing an energy cell pack and a radiation suit (#1). Once you have the radiation suit, you 
can safely walk around the moat to the other side, where there is a teleporter leading back to
 your starting point. At the top of the structure on the east side of the map is a teleporter. 
(To get there, jump down into the slime just east of your starting point and walk north through 
the little door, up the steps (shoot the wall that's in your way), and jump down from the window.
 Then hit the switch to lower the lift to the entrance of the structure, and fight your way up to
 the top.) The teleporter takes you back to the starting point. Just to the left of the 
teleporter is a hidden door which takes you through a small room (#2) with another teleporter, 
which goes to a tower on the north side of the structure. In all likelihood, you'll teleport 
right on top of a pain elemental, killing it before it has a chance to fire any of those damn 
skulls. There are two towers west and east of the circular area to the north of the main building
. There's nothing on the east one, but the west one (#3) has a megasphere on it. To get there, 
take the big demon teleporter in the circular area. Walk down the stairs and kill the bad guys. 
(Note that just south of the stairs, down in the slime, is a teleporter leading to the area 
where the cacodemon was hiding. There's blue armor and some health up there.) Then jump across 
to the dark area east of the stairs. Instead of walking south to the green armor, find the hidden 
k to the teleporter, which takes you to the megasphere. The structure west of the circular area 
to the north of the main building (not #3--the thing to the northwest of it) has a supercharger 
(#4) and a BFG9000 in it. To get there, grab a radiation suit if you have one, and walk northeast
 through the slime to the building with the teleporter to #3. Walk around the southeast corner 
of the building, looking for a staircase; there should be some flaming skulls there to mark the 
place. Walk up the stairs to the teleporter, which takes you to the structure to the west. Hit 
the switch to raise the platform, and kill all the bad guys. If you don't see the BFG9000, shoot
 your pistol out the east window at the big demon teleporter, and a door behind you should open
 revealing the BFG and some ammo. Then hit the switch on the south side of the room to reveal 
the supercharger (#4). Unfortunately, there's a teleporter in the way, which takes you back to 
the demon teleporter; fortunately, there are two secret doors to the left and right of the 
teleporter, which you can take to get to the supercharger (through another secret door to its
 west or east). The left one has an arch-vile in it (#5), so I suggest you take the right one, 
which contains an energy cell (#6), unless you want 100% kills and secrets; but if you didn't, 
why would you be reading this? In the room with a staircase leading to a plasma gun, there is a 
secret door on the south wall, leading to a small room (#7) with a supercharger on the west end. 

Level 21: Nirvana 
No official secrets. To get the megasphere, hit the switch next to the teleporter which takes you
 to the room it's in, then jump in the teleporter and run and get it. Note that the room with the
 arrow-shaped staircase has some weird effects on the ceiling. 

Level 22: The Catacombs 
In the northwest corner of the starting room is a hidden door leading to a room (#1) with rockets
, a supercharger, armor, and a spectre. In the southwest corner of the starting room is a hidden 
door which leads through a short hallway (#2) to the room you can see to your south, containing
 the plasma gun and an army of chaingunners. To open the door, hit space on the green torch. A 
bit northeast of the red skull key is a secret door leading to a small opening (#3) with a 
teleporter, which takes you to the room south of your starting point which contains a plasma gun,
 in case you hadn't already gotten it. 

Level 23: Barrels O' Fun 
Between the two north-south hallways which lead outside is a third hallway containing a 
megasphere and two boxes of rockets (#1). It has a little outcropping just north of it which you
 can jump down onto, and then walk south to get the goodies. In the northeast part of the map 
are two hallways with boxes and boxes of rockets, for use against the various spiders. In the 
south hallway is a door to the north, leading to a supercharger (#2). 

Level 24: The Chasm 
Your starting point is located in a small opening in the north side of a building. The main
 entrance to the building is around the walkway a short distance to the east, but you can't get 
in, because the entrance is too steep. To raise the stairs up, first fall into any of the 
teleporters in the slime on the floor of the southeast corner of the map, north and south of the
 building you're trying to get into. You should appear south of a series of tower. Jump down from
 tower to tower, working your way east to a small area with a teleporter. That will take you to
 another teleporter. Go straight (east) and hit all the switches in that area. That should raise
 the stairs at the main entrance of the building, so get back there by going back to the 
teleporter, and go through it to the teleporter at the base of the series of towers. Then run 
west to the teleporter on the acid floor. That will take you back up to the south tower, so run
 northwest to get back to the building near the starting point. Once you get inside (#1), you'll
 see three things sticking out of the north wall. Hit space on the first one you come to; it will
 lower a lift to your south, which leads to a small area with some ammo (#2). This is the 
invulnerability in the west part of the map, which is not that hard to find; just follow the 
annoyingly thin walkway west. You can jump down onto it, but then you'll be stuck there; you'll
 have to get off that platform and wait for the floor to rise up, then walk to one of the 
surrounding walls, and hit space until you get back up to the level of the walkway. At that 
point, the invulnerability will have worn off. A better idea is to get a radiation suit and 
jump down into the moat around it, and then hit space on the surrounding walls until it reaches
 the level of the platform the sphere is sitting on, then grab it; then the floor will raise up 
to the level of the surrounding walls, and you can get out. Or you can try jumping down just to 
the left or right of the invulnerability sphere, enough to get the floor to raise up and take
 away the poison; then grab the sphere, and hit space a few more times to get back to the walkway
. The south area of the map has a shotgun with a small walkway to its south. If you get the 
shotgun, an opening will appear to the south revealing some monsters. After killing the monsters,
 you can jump down into the opening to get some ammo and health (#4). When you jump back, the 
floor around the shotgun will rise up and become non-toxic. 

Level 25: Bloodfalls 
There are no official secrets, but there are some hidden areas. In the north-south hallway with 
two windows on each side of the hallway, in the north part of the map, there is a secret door 
between the two windows on the east side, leading to a rocket launcher and some ammo. In the 
south part of the map, in the room with some steps leading south between two windows, there is a
 hidden room in the northeast with a BFG9000; it's on the automap.