To get the blue key on level 18, SPAWNED FEAR:

First, go into the room where the blue key is and take a
look at the key.  If there are three short vertical bars in
front of the key, blocking access to it, then see REMOVING
THE BARS, below.  Otherwise:

Just outside the door to the blue-key room is another room.
One wall of this room is lined with what appear to be five
switches in a horizontal row.  The switch closest to the
blue-key door will drop the pedestal that the blue key sits
upon.  You can activate this switch one of two ways: 1) You
can stand on the raised platform next to the blue-key door,
and shoot the switch with your pistol, shotgun (either one)
or chaingun; or 2) Simply walk up to the switch and punch
it, or buzz it once with your chainsaw.
This causes the blue-key pedestal to drop down to floor
level for a few seconds; then it will rise back up again.
In the meantime, you can grab it.
Before you activate the switch, it's advisable to have the
door to the blue-key room already open.

If the blue key is blocked by vertical bars, you'll have to
lower the bars by activating a switch, which is located a
few rooms away from the blue-key room.
First, from the blue-key room, walk out the door (into the
room with the 5 switches on the wall), then walk out the
next door on your right.  As you walk out this door, hug
the right-hand wall and keep walking, past the first door
you come to, then look for the next door.  It'll have a
switch just outside it, to the left.  Open the door and
you'll see a chamber ahead of you, with another door on the
far side, across a square pool of green slime.  Activate
the switch next to you (next to the door you just opened),
and this will open the door on the other side of the slime
pool.  Kill the monster that comes out of the far chamber,
and a floor/grate will rise up out of the slime pool,
allowing you to cross it unharmed.  Once you're across the
slime and in the next chamber, you'll see a switch ahead of
you.  Activate this switch to drop the bars which block
access to the blue key.

Hope this helps. :)                                                

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