Quake 1: All Secrets

Episode 4: The Elder World

Map e4m1: The Sewage System (4 secrets)

Riding the elevator up, on the way to the gold key, shoot this monitor for yellow armour and Quad Damage.
To get the super health pack and Quad Damage you can see on the left, RUN up the slope and jump at the top to head-butt the flickering light. A secret door opens right next to you.
You will find a room overlooking the exit slipgate. A target is visible over the door. Shoot the target, a room opens outside, containing nails and a slipgate back to the gold door.
After opening the final floodgate, you enter this flooded room. The pipe right in front of you leads to the exit slipgate. Check the top area on the left. You'll find a dry room with health and shotgun shells.

Map e4m2: The Tower of Despair (5 secrets)

When you enter this level, you land in front of this stained glass window. Shoot it, and a knife switch appears under it. Push the switch, and the cages behind you open. The double-barrelled shotgun is the secret.
Out of the room, to the right. At the end of the corridor, the foot panel is missing. Shoot here - it's a hidden door. Nails and a Ring of Shadows inside. Don't run outside yet, wait until the door shuts.
When the secret #2 door opens, it glides into a recess in the wall. When it's shut, you can enter that recess. Push the knife switch there, get out of the hole and shoot the door to open it again. Outside the door a hole has opened in the floor. It looks like blue armour, but it will teleport you on top of the big wooden beams in front of secret #1.
Get on top of these beams, and inside the hole in the wall. You can jump right down on top of a super health pack from there. Jumping outside the cage from there will yield Quad Damage and a route to the silver key.
The slipgate from the silver key will teleport you to the platform in front of the stained glass window. Shoot the window and jump into the hole that opens. A lift will bring you up to this concealed chamber. Push the knife switch, and swiftly move away. If you don't move, when the grating retracts into the wall, you'll land in front of monsters. If you do move, shoot the differently textured bit of wall at the back of the room. Then you have the drop on the bad guys. When you jump down from here, the secret counts.

Map e4m3: The Elder God Shrine (3 secrets)

When you arrive here, go inside the right window. Health inside, and it counts as a secret.
See that golden door? Don't go that way. Go to the right and keep going till the very end of the winding corridor. There is a big button on the wall - when you press that, the floor drops down and you have found a secret: ammo, blue armour and Quad Damage.
Now that you are down, jump onto the wooden platform, and down again in a hurry. When it rises, a slipgate is found underneath. It leads to red armour, health and nails.

Map e4m4: The Palace of Hate (5 secrets)

Shoot the sun picture over this arch. The wall opposite with the stained glass window opens, freeing a monster and a Pentagram of Protection.
Just beyond the arch with the sun picture (the same one you shot to get secret #1) is a healing pool. It counts as a secret if you jump in.
In the dark corridor beyond the healing pool is a lift. Ride it halfway up, then step off and dive under the lift. The red armour is yours.
Don't attempt to squeeze under the guillotine blade. Go to the far wall. When you try to pick up the nails, the walls left and right will drop down. Ride the left wall up. After a couple of seconds, a chamber opens, containing Quad Damage and a slipgate to the other side of the guillotine.
Grab the silver key and Pentagram of Protection. Either rocket jump from the hole in the floor or roll a grenade into it and wait for it to explode. You will get health and a Ring of Shadows.

Map e4m5: Hell's Atrium (5 secrets)

Right inside the gold door, step on the floor switch to activate the lift. Get on the lift, and ride it up. Shoot the red target, and a column rises from the floor. Now you can jump across to get ammo, a super health pack and Quad Damage.
The next 3 secrets are underground. Once you step on the floor switch in this room, the stained glass will shatter, releasing zombies, Quad Damage and ammo. One recess does not have any loot in it. Shoot the wall here to raise it. Ride up the lift beyond it. To your left will be an acid pool. Jump in for cells, a Pentagram of Protection, and yellow armour.
Once out of the acid pool in secret #1, go left along the narrow ledge. Jump across and go all the way right, staying up on the ledge. Make your way to the room directly opposite from you. There is a power cell in the room.
From secret #3, you arrive at a corridor atop roof beams. Jump down, go around the corner and use the torch stone to jump on to the roof beam to its left. Walk across to the power cell, which is the secret.
If you use the silver key and lower the bridge, only the exit will be open. But if you jump from stick to stick over the lava, another silver door shows itself, just to the left of the exit. This leads to the last secret level in the game: The Nameless City.

Map e4m8: The Nameless City (4 secrets)

Through the corridor under the golden key is a switch on the wall - too high to reach. Use the small corridor opposite the switch to get a floor up. Run and jump to hit the switch. A door opens a bit further down the corridor. In the end, you'll end up in a room with a Shambler. There is a hole in the floor, drop down and you can get to a red armour.
When you jump down from the Red Armour platform, you are on the same floor you used to get to the switch in #1. One of the holes in the floor is a deep well. A few feet down you'll find a ledge and some zombies. Shoot the bit of wall that has the floor texture to open a room to health and ammo. Jump down to the next ledge, about halfway down. That's the secret: more health, cells and Quad Damage.
Once you get the gold key, for some reason entering the gold door the end of the yard counts as a secret. There is health and cells in the room.
This is right inside the silver key door. Drop down through the triangular hole to pick up a super health pack.

Map e4m6: The Pain Maze (4 secrets)

In the first room you drop into, look for a switch on the rear side of the columns. A room opens out to ammo, one altar, and the silver key.
Jump in the crescent pool. Just before the long corridor to the half-flooded room, go up and find health, rockets, a thunderbolt and biosuit.
First get to the marked beam. Shoot the big roof light, and the beam you stand on elevates, giving access to the red armour.
Secret underwater room - shoot the different-coloured wall. You'll find nails, cells and a Pentagram of Protection.

Map e4m7: Azure Agony (9 secrets)

This Quad Damage sits on a trap door that will drop you in acid, right in front of the slipgate out. FIRST get the biosuit from the nearby hallway, then activate the trap and fall into the acid. Do not take the slipgate. Run around the corner, a second slipgate will transport you to red armour. Pressing a switch on the red armour platform will yield some ammo.
Underneath these stairs is a switch. Press it and take the slipgate back to the staircase. Take the stairs down to the red brick room - a secret room is open now with some monsters and ammo.
This is a long series of secrets. It starts in this room. Shoot the wall under the Pentagram of Protection. Secret room with rockets...
...Then shoot the wall under the Quad damage. A door slides aside and gives you access to a secret slipgate...
...Actually entering the slipgate is yet another secret! It sends you to a ledge with a monster, super health pack, Ring of Shadows and ammo.
Jumping down from the ledge counts as yet another secret! You can alternatively get to the lower part of this platform secret by shooting this switch. Stand on the extra-wide step, and you can ride it up when you shoot.
Before jumping down the ledge, notice the slit on the left? That's a door you can shoot to open. Entering the secret room is one more secret. You'll find Quad Damage here.
Straight across from the ledge two spotlights shine on otherwise dark walls. Shoot the left one for rockets...
And the right one for even more rockets!