How to find the secret exit in Original-Xbox Ultimate Doom
level E1M1, Hangar, which leads to level E1M10, Sewers:

Start a new Ultimate Doom game on Episode 1, Knee-Deep In
The Dead.
When you appear on the first level, Hangar, you'll be at
the south end of a room, and you'll be facing north.
Walk to the northeast corner of this room, and enter the
hallway there.  Follow it to a door, and go through.
Go through the next room, to its southeast corner, and step
into the short, curved hallway there.
Follow this hallway to a large room, where a zig-zagging
walkway leads south through a wide pool of green slime.
Go south, to the far end of this room, where you'll see a
door at the end of a straight hallway.
Just before you get to that door, search the right-hand
side of the hallway for a secret panel.  When Activated, it
opens like a door.
Walk through the open panel, and you'll be in a tunnel that
leads outside to a small courtyard.
Go to the east side of this courtyard, to the single, large
rectangular window.  Then turn left and walk north a few
paces, to where the edge of the window meets the north wall
at a corner.
Follow this wall a few paces to the west, until the wall
begins to turn northwest.
Activate this diagonal section of the wall like a switch,
and you'll exit this level and enter E1M10, Sewers.