How to get the keys in Doom II level 13, Downtown:

(Note: To view a video demonstration of acquiring the keys
on this level, click here.)

BLUE KEY
--------
There's a couple of different paths you can take to get to
the blue key.  Here's one of them.
Go to the gray brick building in the northeast corner of
the map.  The bottom half of the building has no walls,
just thick pillars at each of the four corners.  At ground
level, centered between the pillars, are four teleport pads
arranged in a flashing square.  Step on any one of them.
You'll appear on the southwest corner of a raised ledge
that runs around the top of a building located near the
center of the map.  You'll be facing north.
Turn 90 degrees to face west, and put your back against the
low wall that's now behind you.  You'll be facing towards
one of the black buildings; it's southeast corner will be
in view.
Run forward (west) off the ledge you're on, and you'll land
on another low ledge that runs part way around that black
building.  (Run in PC Doom games is typically enabled by
holding down the Shift key.)
On this ledge you're now on, you'll have access to a metal
door near the southeast corner of this building.  (This
ledge also leads north, then west, to a backpack.)
Step through that metal door, turn right, and go down the
steps ahead.
You'll be in a room which has a locked wooden door in the
northwest corner, and a metal door in the southwest corner.
Go through the metal door and then follow the short passage
south and east, as far as it goes.  When you get to the far
end, you'll cross an invisible trigger line that will open
that wooden door back in the other room.
Go back to that room, and head towards where the wooden
door was.  You'll see the blue key there on the floor, and
behind it are a couple of teleport pads.  Both pads will
deliver you back to the level's starting point, near the
southeast corner of the map.

RED KEY
-------
In order to get the red key, you'll first need the blue
key (see above).
There's a couple of different paths you can take to get to
the red key.  Here's one of them.
Go to the gray brick building in the northeast corner of
the map.  The bottom half of the building has no walls,
just thick pillars at each of the four corners.  At ground
level, centered between the pillars, are four teleport pads
arranged in a flashing square.  Step on any one of them.
You'll appear on the southwest corner of a raised ledge
that runs around the top of a building located near the
center of the map.  You'll be facing north.
Turn 180 degrees to face south.  You'll be facing towards
a light brown building; it's northwest corner will be in
view, just left of center.
Run forward (south) off the ledge you're on, and you'll
land on another low ledge that runs all the way around the
bottom of the brown building.  (Run in PC Doom games is
typically enabled by holding down the Shift key.)
On this ledge you're now on, you'll have access to a metal
door in the middle of the west side of this building.  You
need the blue key to get through this door.
Go through the door and turn left.  Drop down into the room
and go to the red teleporter on the floor in the northwest
corner, and step on.
You'll appear on an upper level of the building that is
just west of the building at the northeast corner of the
map.  You'll be facing south, and the red key should be in
view, just ahead and to the left.

YELLOW KEY
----------
The key is located in the building at the southeast corner
of the map.  You'll need a red key to gain access.
Walk through the entrance of this building and approach the
tall lift you'll see ahead of you against the east wall.
As you approach, it slowly drops down.  Once at the bottom,
it'll pause for a few moments, then begin to rise again.
Get on and ride to the top.
As you're riding up, turn and look to the northwest.  You
will eventually see the yellow keycard come into view...
It's at the top of a stair-stepped structure that's against
the north wall.
When the lift has taken you all the way to the top, back up
a pace or two to get a running start, then Run to jump to
the yellow key.  (Run in PC Doom games is typically enabled
by holding down the Shift key.)

PC Doom II players can click here to access a demo that
shows how to access all the keys on this level.