How to access the keys and the exit in MEPHISTO.WAD:

To finish this level (without cheating), do not use the
"-nomonsters" command-line parameter when starting the
game; Monsters must be present.

To exit this level, you'll need a red key and a blue key,
and then you must destroy a "Boss" monster at the end of
the mission.

You begin the mission in the northeast corner of a large
walled-in courtyard.  You'll be facing southwest towards a
large squarish building, which dominates the central region
of the courtyard.
Watch out for Revenants patrolling the area.
In front of you is a backpack, and just beyond that is a
shotgun; grab them.  Shotgun shells can be found on the
ground nearby to the east.
There is a chaingun, boxes of bullets, and a Berserk Pack
in the northwest corner of the courtyard.  You can also
find a few Medikits located around the level.
In the southeast and southwest corner of the courtyard you
can find boxes of rockets.  You may want to grab them now,
if possible.
There are also other boxes of rockets, individual rockets,
and some backpacks scattered around the level; many of them
are inside the large central building.

WARNING: You'll want to have a minimum of 10 rockets in
your inventory once you approach the end of this mission;
Be sure not to fire all of them off before you confront the
Boss monster.

RED KEY
-------
Note: You'll need the red key before you can access the
blue key.
The courtyard grounds are dotted with trees, tree stumps,
and four-sided, flat-topped columns, or "obelisks".
Look for a large wooden door in the middle of the west side
of the large central building.  Once you're at this door,
look west for a nearby obelisk.  It will be a slightly
different color than all of the others.
Step up to this obelisk and Activate it like a switch.  The
obelisk will telescope down into itself, revealing a red
teleporter pad on a base (and another Revenant, waiting on
top of the teleporter).
When you step up onto the teleporter, you'll appear in the
center of the main building, facing north.  Surrounding you
are four huge demon-faced walls: one each to the north,
south, east and west.
You'll be standing on a wide, raised pedestal in the center
of the building.  Four bridges lead away from the pedestal,
forming a "plus"-shaped pattern, with the pedestal at the
center.  Each individual bridge leads to a wide platform
running along the base of each of the four demon-faced
walls around you.  All of these platforms are also linked
at their corners by diagonal bridges.
Watch out for Revenants on these bridges and platforms.
(Note: On skill levels 1 and 2, there is an Invulnerability
Artifact located on the south bridge.)
If you fall off of a bridge or platform, look for a nearby
teleporter pad on the ground (which will return you to one
of the platforms or the central pedestal), or look for one
of the diagonal bridges, which can be lowered by activating
it like a switch (however this will only work on "interior"
sides of the diagonal bridges, the sides facing the central
pedestal).
Raised up high in the center of each "demon wall" is a
rectangular, barred window.  Behind each window is a small
chamber, and in each chamber is a Mancubus monster.  Each
Mancubus must be killed before you can exit this building
or access the keys.
(You can find various weapons on each of the platforms
beneath the "demon walls", including a rocket launcher on
the north platform.)
When the last Mancubus is killed, the four corners of the
building will open up at ground level, releasing Hell
Knights and Arachnotrons.  Once these monsters begin
moving, they may be teleported: Hell Knights tend to be
teleported to positions up on the bridges and platforms,
Arachnotrons tend to be teleported up into the chambers
behind the "demon walls", where the Mancubus monsters were.
A red skull key is now accessible.  It's at ground level,
resting on a corrosive floor in the northwest corner of the
building.  (You'll also find 2 boxes of rockets there.)
In the southeast corner of the building, at ground level,
is a door that can be opened with the red key.  Exit the
building through this door.

BLUE KEY
--------
Note: You'll need the red key before you can access the
blue key.
Once you have the red key and have exited the large main
building, look for a small, separate building in the
northwest corner of the courtyard.  Watch out for Revenants
teleporting into the courtyard.  As you approach the small
square building in the northwest, use the red key to gain
access through a door in its southeast corner.
The interior is a dark, squarish room, with a coffin-like
object in the middle of the floor.  Look for a switch in
the northwest corner of the room, and activate it.
Warning: In the center of the room, another Revenant will
begin rising up out of the "coffin".
Head back to the courtyard, and go to the outer southwest
corner of the large central building.  You'll see some blue
Mega-Armor behind a post at the corner of the building.
Step behind the post and pick up the armor.  When you do
this, some stair steps will emerge from the ground, just
outside of the south side of the building.
Follow these stairs all the way up to a door at the top.
Open the door and take one step inside.
You'll be at the east end of a small three-way junction,
composed of a short east-west corridor (now open at both
ends), intersected by a short passage leading north.

There are 2 invisible floor triggers, or "tripwires" here:
One running north-south across the floor directly in front
of you, and another running east-west across the end of the
opening of the passage that leads north.
When you cross the first tripwire, a box-shaped corner of
the intersection opens up ahead of you; Crossing the second
tripwire at the corner of the intersection will trigger an
event in the small building at the northwest corner of the
courtyard.  (A "stair step" now rises and falls repeatedly
around the outer edge of the "coffin" in that building.
This allows you to climb up on the coffin.  From there you
can Run north to get into a small cubbyhole in the north
wall.  Once inside you can open a panel in the west wall of
the cubbyhole to reveal an Invulnerability Artifact.)

In the 3-way junction, go north to step into the back of
the chamber behind the south-side "demon wall".
In this chamber is a tall rectangular window.  From here
you can look north, out and down upon the interior of the
main building.
In the northwest quadrant of the building, you'll see a
tall pillar, located in the middle of the "pit" which is
formed by the bridges that surround that pillar on all
sides.  On top of this pillar rests the blue skull key.
You can walk out through this window ahead of you, and drop
into the main building, or simply walk to the very edge of
the window, without stepping all the way through.  When you
do this, another invisible "tripwire" is triggered.  This
causes that tall pillar in the northwest quadrant of the
building to drop down to ground level.
Get down into that "pit" in the northwest quadrant to get
the blue key.

TO EXIT THIS LEVEL
------------------
Once you have both keys, get back out to the courtyard.
(You can exit the main building from an open door at ground
level in the building's southeast corner.)  Go to the west
side of the courtyard, and look for a door between two blue
torches on the west side of the main building.  Open the
door to reveal a small chamber.  Inside are a pair of
Baron-Of-Hell monsters, and a Megasphere.
At the center of the back wall (east wall) of this chamber
is an elevator in the raised position.  Before you activate
this elevator, be sure you've collected as many rockets as
possible.  A minimum of 10 rockets is recommended at low
Skill levels; more at higher Skill levels.  You may also
want to acquire the Invulnerability Artifact in the small
building in the northwest corner of the courtyard, if you
didn't use it already.
Activate the elevator to drop it, then ride it up.
The elevator comes up in the back of a wide chamber which
is behind the west-side "demon wall".  There may be one or
more live monsters here.
In the east wall of this chamber is a tall rectangular
window.  From here you can look out upon the now-darkened
interior of the main building.  In the distance you'll see
the east-side "demon wall".  Behind that distant wall is a
chamber similar to the one you now stand in.
When you step off the top of the elevator you rode up on,
the back wall opens up in that distant chamber on the east
side of the building.  Behind that wall is a crowd of
immobile Spectre-Demons, and behind that is the level's
"Boss" monster.
When the Boss monster detects you, it will begin launching
small, brown, skull-faced cubes through the air towards the
west side of the building where you are.
These cubes will fly through the air (and can pass through
solid objects) until they reach one of three possible
destinations: 1) the north side of the chamber you're in,
2) the south side of the chamber you're in, or 3) an
outdoor spot in the courtyard, to the west.  When a cube
reaches its destination, it spontaneously transforms into a
monster.  The monster may be of any type, excluding
Cyberdemons, Spider Masterminds, Arch-Viles, Lost Souls and
humanoids.
To win the level you must kill the Boss monster.  The
faster you do this, the better, as the level will begin to
fill with monsters.
From where you stand in the chamber behind the west-side
"demon wall", use your rocket launcher to fire rockets
across the open interior of the main building, through the
window in the east-side "demon wall" and to the back of the
chamber where the Boss monster is.
You'll need to use enough rockets to penetrate through the
cluster of Spectre Demons, to the Boss.  10 or so rockets
through the distant window should do the trick.
When the Boss monster is killed, the mission ends.

To read about or download a demo of this level, click here.