This level can be accessed through the Holding Area. Near the end of the level, you will see 4 switches. Flick them in the following order: 3, 1, 2 and 4. The circular platforms ahead should now line up. Now go through the small door and get into the teleporter.

Now you will appear in line with the 4 circular platforms. Run across them and go into the red room. Kill the Nightmare Imp and exit the stage. This is a Secret Exit which will have you go straight to Outpost Omega. Be warned that you will only have the guns that you exited the Holding Area with.