How to get the blue key in level 20, BREAKDOWN:

On this map, you'll first appear in a small 3-sided chamber
that is centered in one of the short walls of a rectangular
courtyard.  On the opposite wall is a stairway leading up
to a raised path that runs all the way around the perimeter
of the courtyard.

In the middle of the courtyard is the blue key, perched on
a high, narrow pedestal.

There are two tall, narrow vertical columns very close to
the blue-key pedestal: one column each on opposite sides of
the pedestal.
High above ground, on one side of each of these narrow
columns, is a small switch that faces towards the blue key.

Walk up the stairway and turn left.  Walk a few paces
around the corner, until you get to the middle of the part
of the path that runs along one of the longer walls of the
rectangular courtyard.

Turn to face towards the blue key.  Check your map to see
that the map's entry point (where you started the game) is
to the east, on the right-hand side of the screen as you
face towards the middle of the courtyard.  If this is not
the case, run around the pathway to get the opposite side
of the courtyard.

Look towards the blue key, and maneuver so that you can see
the switch on the column just beyond the key.

Use your pistol, shotgun (either one), or chaingun to shoot
the switch.  The blue-key pedestal will drop, and you can
then grab the key.

(If the switch on the other column is shot, a Cacodemon is
released nearby.)

Hope this helps. :)                                                

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