DOOM - EPISODE 1
KNEE DEEP IN THE DEAD 

E1M1: HANGAR: 
# Just after the zigzag look right for the oddly colored wall. Push it for corridor to a open place with a blue armor vest. 
# After you cleaned out the exit room, go back to the zigzag room and look right for the shotgun (the tower with the Imp should be lowered now) 
# For DOOM v1.2, a secret area has been added in the above secret room: look in the SE corner for the elevator. You'll find some armor bonuses up there. The elevator lowers when you are at the entrance in the north, so you'll have to run fast to get it. 

E1M2: NUCLEAR PLANT: 
#The small structure you face when you begin, has a secret door in the SE corner (the lighter wall part). Behind the door is a room with a red switch. Pushing that switch will open a secret door in the northern corner of the stairs in the east. You can get outside and collect a supercharger and a chaingun. 
+ Behind the red door you'll see a stair twisting to the right. On the top is an indention to your left. If you step on it, it will drop down. You will find enemies to your right, and you will also see (but not be able to get) the chainsaw. 
# A bit ahead you'll see a lift. Push the first panel to your left. You'll be able to kill those Imps from the ridge. 
# In the southern part of the maze (to get in, pull the switch behind the pillar in the green toxic directly to the right behind the red door, shoot the middle most western wall (the one behind the green armor vest) for access to the chainsaw (push the switch in NW to lower the pillar so you can get it). In the same hidden area is some extra armor hidden behind the eastern wall in the northeastern corner of the stairs. 
# In the most eastern part of the maze, push the wall between the white stripes for a backpack. 

E1M3: TOXIC REFINERY: 
+ The strange grey wall straight ahead beyond the first door you open is a secret drawbridge. The switch for the drawbridge is in one of the chambers described below. 
# In the room in the NW (the one that opens as soon as you push the red button and that contains the yellow key) are TWO secret passages (the second niche on your left and the second niche on your right, both will open when you're on the stairs but will close again after a short while. Go to the stairs and then run like hell as soon as you hear them lower. It'll help when you shoot the drums that might get in the way. 
# In the northern niche (leading to a rocket launcher and a green armor) is a secret room in the SE corner (just follow the green stuff to the right). Run fast for minimum damage. You'll see a backpack and the switch for the drawbridge at the beginning. To the east is the exit (a lighter colored wall part) through another secret chamber containing the chaingun. 
# In the south-eastern niche (the second on the right) is a supercharger (pull the switch on back of the pillar). 
# The drawbridge should be lowered now. Cross it, but be careful! There are a lot of Imps lurking behind automaticaly opening doors. My tactic is to select the chain gun, run forward and back up as soon as you hear the doors open. Then take them out at your leisure. Note that you can also shoot some of them via the secret `window' in the SE. 
+ In the north of the room is a corridor, follow it to find the hidden-level- switch. 
# To the north of the room with the hidden-level-switch is a elevator, push the wall to lower it, so you can get the rocketbox. 
# Behind the blue door in the eastern corner is a secret door (the lighter patch in the SE. The chainsaw, a supercharger, a blue armor and a partial invisibility is available. 

E1M9: MILITARY BASE: 
You start facing a open space with a cage full of Imps in the middle. The following rooms are described relative to that cage. 
+ In the northeast is a pentagram on the floor. If you touch it, numerous enemies will warp in. Try walking around it and then running to the corridor. That way you can pick them off one by one. 
+ In the eastern room is a lift/trapdoor at the middle of the eastern wall (the highlighted patch on the floor). To the NE are a green armorvest. 
# Also in the eastern room is a lift/trapdoor in the SW-corner leading to a indoor lake with the chaingun. 
# The same area is accessible via the lift/trapdoor in the NE-corner of the southern room. 
# In the northern chamber, push the middle eastern wall. You'll get into a chamber with several high pillars. The pillar at the far side of the room lowers when someone is on the stairs. Just run on it, wait for it to rise, and 'jump' to the other pillars by running fast counter clockwise. 
+ To get to the end of level switch, run down the trench. To the NW is a isle with the switch and to the south is the lift upstairs. 
+ Although they don't count as secrets in themselves, getting on the five pillars, each with a weapon or bit of ammo on it, in the secret room on the right hand wall of the room with the ooze trench and exit, is one of the more challenging things to do in the game. Basically, the room on the secret level of episode one "Military Base" contains five pillars of increasing height as you go around clockwise, along with a lift which lowers when you are on the entry stairs, allowing access to the first and highest pillar. The key is to go around each pillar in a counter-clockwise direction. Just running and jumping toward a pillar isn't enough, as you will go over it but fall off the far side, and this can only get you to the first and tallest pillar, and the shortest (or maybe the first two on either end if you manage to swipe the item off the top of the second pillar before falling). The key is to shift+forward run at each pillar, take off and jump toward it, and right when you reach it, briefly press the down arrow to move backward. This will stop your running and you will be standing still on the next pillar. If you move back too soon you will hit the pillar and fall to the ground; too late and you run off the far side. It's best to give yourself a running start for each pillar and line yourself up so you will be jumping over the minimum distance possible. Using the automap under a high zoom level can help here. This actually isn't as hard as it may seem to get right. I understand some can do this all in one fell running swoop by turning in mid-routine, but this is a surer way. 

E1M4: COMMAND CONTROL: 
# The light wall to the left in the beginning (before the door) hides a backpack. 
# The south exit from the round chamber (the one with the radiation suits) leads to a trench. Follow the trench east for the rocket launcher and a supercharger. To get the supercharger, simply stand on the left side of the lift and press the switch. If you are just barely on the lift, you can reach the switch while on the lift. Follow the trench west to get out. 

E1M5: PHOBOS LAB: 
# The chamber in the middle (behind the first stairs you encounter from the start) has a secret chamber to the south (behind the drums) with a shotgun. 
# In of the east of this chamber is a toxic lake. In the SE-corner of the toxic is a secret passage (the strangely colored wall). There is a rocket launcher and a blue armorvest. 
+ In the NW behind the yellow keycard door is a chamber with two pillars and an isle with a switch in the green stuff. In the indention in the west is a secret chamber with the antiradiation suit. The south wall of this chamber hides another room with the chainsaw. The south wall thereof gives away to let you outside and get the supercharger. You can get the blue armor in the NE-corner by stepping on the northern pillar when it's down, waiting until it rises again and then 'jumping' over. Ammo can be found by repeating the same action on the southern pillar. 
# Two platforms with Imps hidden in the walls behind the yellow keycard door act as elevator in DOOM v1.2. The southern one doesn't lead anywhere. The northern one has a secret wall to the east which leads to a secret room with a partial invisibility and a shortcut to the darkroom (via the north). The pentagram is a teleporter back to the beginning. 
# In the north eastern chamber (the one with the blue floor and the blinking light) has a secret chamber to the SE with a backpack, a chaingun and a computer map. 
# In the northern darkroom is a partially hidden chamber to the SW. The best way to get there is to follow the wall left. You'll find the night vision, which is very useful in sniping those baddies in the dark. 
+ In the northern darkroom the top of the /\ in the middle wall is also a secret door, but as far as I can see this isn't really useful (maybe for dashing for the exit?) 

E1M6: CENTRAL PROCESSING: 
+ At the first 'crossing' you encounter, look in the NW corner. There is a partially hidden chamber with some ammo, health and a green armor vest. There are also some Imps there, so to be safe, fire a shot into it to detonate the barrels and kill them. 
# Behind the red door in the southwest is a green lake. In the NE-corner is an anti-radiation-suit. In the 'middle' (the tunnel in the SW) is a supercharger. In the SE-corner is a nightvision, a partial invisibility, a blue armor and the exit from the lake. 
# Just before you are by the blue door there is a secret chamber to the south. Before you're at the blue door, there is a tower with Imps. When you reach the blue door it will lower so you can get in. You will see an anti-radiation-suit. Push the south wall for a backpack, a rocket launcher, a partial invisibility and a back way into the room with the yellow key. 
# In the same area as where the lowering tower is, is a corridor to the north. As soon as you get the yellow key, it will be accessable. The twisting path over the toxic waste will lead to a backpack and a door to the outside. There you will see a supercharger on a pillar. The pillar will lower as soon as someone goes near it. Step on it and as soon as it has risen, press the wall to the west to get inside the complex again. 
+ In the room immediately before the ending room, there is a radiation suit available behind one of the pillars. 
# In the most eastern corridor is a strangely colored wall. Push it to get the computer map. 

E1M7: COMPUTER STATION: 
# In the southwest is a big pillar that will descend when you are on the stairs east of it. Stand on top of it, wait until is has risen sufficiently and jump on the ledge in the NE for the computer map or the south lake in the NE corner for the chaingun and (when you leave via the secret door to the east) a backpack. Jump in the lake in the north and run fastly to the tunnel in the NW. In the west of that tunnel is a radiation-suit and in the east behind the door with the light are some goodies and a switch. Throwing the switch will allow you to get to the chainsaw later on in the room in the extreme SW. Leave the tunnel via the north and look behind the pillar in that lake for blue armor. Follow the acid to the north to the supercharger and leave via the east. 
# After you get the blue key in the northwest, two hidden areas will open to free Imps and former humans. The secret room in the middle of the map has a false wall to the east which leads to outside so you can get a partial invisibility. 

E1M8: PHOBOS ANOMALY: 
# The only secret room is in the east in the corridor leading to the first door and contains a supercharger. 
+ Note that there is a map in the east of your starting point.