Below I have instructions for finding all the secret levels, accompanied
by some crappy, but effective Ascii maps.  It's a lot better than having
to download three stupid demos.  View them with a monospaced font and in a
program that doesn't wrap words...Windows Notepad works fine.

Here we go...

Level 10

Right before you enter the Red Door, there is a secret door on your left. 
It is best to go here first, because you can take out the Vulcan guy
waiting for you before he has a good shot.  You'll see the secret exit, as
well as a grate which is on the other side of the Red door.  Blow the
reactor, then come back here and you're off to secret level 1.

------------------           |XXXXX|  <-- Secret Exit!!!
|                |           |     -------------
|                |           |                 |
|    --------    |           |     --------|   |
|    |      |    |-----------|XXXXX|---X---|___|-------------------------
|    |  UP  |             Hidden^Door  X     ^Window
|    |  to  |    |---------------------X------------          -----------
|    --------    |                  Red^Door       |          |
|  Normal Exit   |                                 |          |
|                |                                 |          |
-------|   |------                                 |          |
       |   |                                       |          |
       |   |                                       |          |
       |   |                                       |          |
                                                   |          |
                                                   |          |
                           ------------------------|          |----------
                           |
                           |
                           |
                           |       |-------------------------------------
                           |       |
                           |       |
                           |       X
                           |       X Hostage Room
                           |       X
                           |       |
                           |       |
                           |       |
                           |       |
                           |       |
                           |-XXXXX-|
                         Reactor Room

Level 21

Since everyone has a different idea of left and right, allow me to
describe my point of view first.  I am on the side of the reactor room
with two doors.  Directly across are three doors, and a door flanking both
sides, as well as a robot generator just behind the side doors.  Once you
blow the reactor, go through the door on the right.  You'll go through an
energy station, and then pass the NORMAL exit, which will be above you. 
Continue through the door, and take the road down into a gray zigzagging
passage.  Once you've come around four corners, turn around and shoot the
corner in the ceiling.  A secret door should open up, revealing the exit
to secret level 2.

                                                                          
                      Reactor Room
                                                                          
                        |XXXXX| <-- Red Door
                                                                          
                        |     |
                                                                          
                        |EEEEE|
    |------------------------|           |-----------------               
                        |EEEEE|<-- Energy
    |                        |           |                |               
                        |EEEEE|   Station
    |                        |           |                |               
                        |     |
----|     |------------|     ------------|     |-----|   
-----------------------------------------|     |
      UP to            |                       |