How to get the Megasphere on level 17, Watch Your Step:

This map is dominated by a pair of courtyard-like areas,
connected to each other by a large, bracket-shaped
("["-shaped) passage.
In the middle of this passage are a pair of locked doors,
one each on the left and right walls of the passage, facing
each other across the hallway.
One of these doors leads to an Invulnerability Artifact in
a small chamber, the other opens to a passage which leads
to the Megasphere.
Here's how to open those doors:

You begin the mission in the larger of the two courtyards.
From here, go to the smaller courtyard through the bracket
passage.  The far end of the passage comes out in one
corner of the smaller courtyard.  A blue skull key rests at
the diagonally opposite corner of this square courtyard.
Pick up this key, and proceed clockwise along the path that
runs around the courtyard, and look for a closed gate on
your right, along one side of the large squarish structure
that occupies the center of this courtyard.
The gate is made of crisscrossing horizontal and vertical
bars, and through it you can see a Pain Elemental at the
end of a short passage.
Open the gate, kill the Pain Elemental, and look for a
switch on the wall behind where the monster was.
Walk up to this switch and activate it.  This will cause a
pair of lifts (one each to your right and left) to drop
down.  Get on either one and ride it up.  When it gets to
the top, step off before it drops again.
Walk around and look for a teleporter pad in the center of
this structure.  You can get to it from a number of
different directions, and aside from a few Imps, it's the
only thing of any importance in this fairly small
structure.
Step on the pad, and you'll be teleported to one end of a
tunnel which runs along the outside of one edge of this
courtyard.  You'll be standing on another teleporter pad,
facing towards the far end of the tunnel.
Don't step off the pad yet.
Behind you is a special door, it's a combination of door
and projectile-activated switch.  At the far end of the
tunnel is a similar door.
Select your pistol (or either one of your shotguns, or your
chaingun), and fire at the far end of the tunnel.  When you
strike the distant door with a shot, the door behind you
opens, revealing a small chamber, with a switch at the
back.  Step into this chamber behind you and Activate the
switch.  (This switch opens the door back in the bracket
passage which blocks access to the Invulnerability Sphere.)
From this small chamber you're in, step back on the
teleport pad ahead of you.  You'll re-appear in the central
structure of the courtyard, on the first teleport pad.
Step off and back on again to re-appear where you just
were, facing down the tunnel.  Now step off the pad and run
down the tunnel to the far end.  At the far end is a lift
that will carry you the short way up to the door you were
shooting at a minute ago.
Right in front of this door is another teleporter.  Step
on, and when you appear at the receiving pad, step off it
and then back on again.
You'll re-appear on the pad you were just on a few seconds
ago, and you'll be facing down the tunnel in the opposite
direction.  Again, use a pistol or similar weapon and fire
at the far end of the tunnel.
When you strike the door at the far end, the one behind you
opens, allowing access to another small chamber with a
switch at the back.
Activate the switch, and back in the bracket passage the
other door will open, allowing you access to the tunnel
that leads to the Megasphere.