Quake 1 Cheats (PC)

Go to the episode 4 entrance and enter the water. Now backup against the wall as if trying
to go back the other way. Stay there until you fall again. You should land on a big piece
of wood. Walk along the side to find another entrance that leads to the Nightmare gate.

To find the "Dopefish," when you see the exit, turn back go onto the bridge. You should
see a quad power-up on a ledge. When you get on the ledge, you will see an extremly small
ledge that you can barely walk on. At the end, you can jump into "The Well of Wishes." The
Dopefish will be there.

To enter the ''command line'' press the ~ key at any time during gameplay. Enter the
following into the lines and press Enter to enable cheats.
  god - Enables God Mode (invincibility)
  fly - Enables Fly Mode
  kill - Instant death
  give S# - Gain # shells
  give H# - Gain # health
  give N# - Gain # nails
  give R# - Gain # rockets
  give C# - Gain # cells
  give # - Get weapon #
  noclip - Walk through walls. Repeat to turn off.
  notarget - Monsters won't attack you unless provoked.
  impulse 9 - All Weapons.
  impulse 11 - Display runes (you don't actually get them)
  impulse 13 - Silver key.
  impulse 14 - Gold key.
  impulse 255 Quad Damage.
  map ExMy - Warp to episode X, mission Y
  skill # - Sets difficulty level (# = 0-3)
  sv_gravity -# - Reduces gravity by #%.

This is pretty strange and I can't really figure out if it is a secret or a bug but here
it is. Type in "Give health". You die but you are still alive. It looks like you are dead
and bad guys can not see you but you can shoot. If your health goes down the slightest you
are really dead but until then you aren't. To move, you put god mode on and get your
rocket launcher out. Shoot the wall and it will richochet in your favored direction.

This is even harder than the hidden Nightmare skill. To reach it, follow the instructions
for the Nightmare difficulty, but when you start down the hallway to the portal before
you turn, there will be a wall decoration type thing. Shoot this with your shotgun, you
will know if you did it because the wall will bleed and a message saying you have
selected Crypt of Decay mode pops up. Then proceed down the hallway to the portal and
choose your episode.

This code has been tested with version 1.06. Beat any episode of Quake and save as much
ammo and armor as possible. After you return to the level select screen, save your game
and quit out of the game entirely. Now re-enter the game and open that save file. Walk
backwards into the teleporter and return to the first telepad (where you start the game.)
Go toward the Hard difficultly and fall into the lava. When you restart, you will respawn
with all your health, ammo, armor and weapons! You will lose your rune but that's alright
compared to the power you have to enter later episodes.

In episode 1 at "The Grizzly Grotto", after you hit all quake switches after going up the
elevator, doors will open to your left and right. Enter one and hit the switch. Do the
same thing again on the other side. It will say "A secret cave has been opened." Go to
the water. when you enter, look around. You will see a cave opened where a portal leads
you to Ziggurat Vertigo.

To find The Underearth level you must be in The Crypt of Decay. When you see the exit, go
straight back. You should be in a room with boxes and a button on the ceiling. Shoot the
button, then jump on the boxes into the secret room. Push the button, then go to the
hidden "Well of Wishes." Now look around the walls and a wall will open for you. When you
get out, there will be a underwater doorway that leads to the secret level.

When you enter the room with Shub-Niggurath you'll see a spiked ball if you play through
the level. When it goes through Shub-Niggurath, walk through the teleporter on the
walkway.

F10 - God Mode
  F9 - No Clipping Mode (C and D keys move up and down)
  F4 - See Through Walls
  9 - All Weapons

To find the weird picture, find all the faces on the walls and shoot each once. You will
know which ones you shot because blood will come out only the first time. When you shoot
all of them, look for a hallway with a short opening and a villain on each side. One of
them will open to a small room with a picture in it.