Quake/The Crypt of Decay

Head up the lift, and take the right exit. Press the button to extend the bridge to the central tower and cross to press a button that opens a nearby door (behind the window).

Loop back around to the bridge (or jump in the water and take the red armor secret) and go through the door. Cross the bridge and take the right exit. Press the button to extend the bridge and cross the newly extended bridges.

After the second bridge, there is a spike trap. Walk slowly towards it from the side; once it springs, run through it before it recharges. Take the gold key, and return back to the gold key gate (there's only one path - barriers block most other pathways aside from taking a dive.)

Open the gold gate, and proceed onward. You'll soon come to the spike corridor where nails will shoot from the walls; on easier difficulties, only some of the shooters fire their shots, and do so less frequently.

Across the bridge, the exit to The Ebon Fortress is there.

Weapons[edit]

Nailgun: Appears near the start, before the lift.
Super nailgun: Past the gold gate.
Grenade launcher: In the yellow key room.

Secrets[edit]

Jump into the water near the entrance or the following room(s). Follow and swim in the direction of the arrows, and you'll eventually come to a teleporter with red armor.
Near the yellow key gate, there's a zombie on a outcropping. Kill the zombie to open a door left of the yellow-key gate. This opens a passage that leads said balcony.
When you pick up the yellow key, you get ambushed by monsters. Shoot into the right alcove to activate a secret door and yellow armor. The other part of the secret is blocked by an underwater barrier.
- You can also reach this secret through the Well of Wishes.
Near the exit, you should see a ceiling button located in the opposite direction of the exit. Shoot it to open a secret door between a few boxes, and to lower an underwater barrier in one of the secrets. Return to that place to exit to the secret level, The Underearth.

The following applies to the map, but does not register as a secret:

When you first climb the lift and start to cross the bridge, you hear a door open behind you. This is in the entrance room, and reveals a few nail boxes.
When you reach the bridge leading to the exit (from the spike tunnels), there are alcoves that you can jump onto, and are located on both sides of the spike corridor. The left one contains some rockets, the right one contains Quad damage.
From the Quad damage alcove, you should see a ledge on the left. Follow it carefully to reach the Well of Wishes. You can find a biosuit there.