Quake/The Necropolis

Entrance
This level will be the first time you see the grenade launcher. Pick it up, and use the first grenade take out the group of three zombies. When you enter the next room, you will be flanked by two fiends that will quickly attack. Run back to avoid their attacks, and finish them off with the shotgun. Return to the previous room, and take a left turn.

This cylindrical room contains a nailgun. Picking it up triggers a crushing ceiling, but you can open some stairs by hitting the button. The stairs reveals a tunnel that leads towards a large room. Approaching it will release zombies that attack you from behind, and scrags that attack from the main room. Jump down the pit below and get the gold key. This releases zombies from underwater, which can be blasted by grenades as necessary. Head through the corridor to a lift, which you can use to return to the starting area.

Head to the stairs and back to the main room. Either run past the Shambler, or defeat it with the nailgun (with a short delay before the second appears). Then, enter the gold key gate.

Past gold gate
After a short run, you will encounter an apparent dead end. Approach it, and a mechanism will trigger. Stand to the side and let it finish opening the trap door.

Once you fall in, there will be zombies hiding in the water. Use grenades to take them out. If you want to return to the previous part of the map, take the left-hand turns. Otherwise, proceed to the right.

When you leave the water, you will encoutner three ogres - one attacks from the front, and the others from the ledge above watery tunnel. The ogres above guard a button which opens the four doors in the room and releasing upto four zombies (and gives access to some items.)

Exiting
Past the door, and to the left will be a fiend. There will also be two ogres trying to throw grenades from above, which may be killed to get access to the yellow armor. At the end of the corridor will be another cylindrical room with a button. Press the button to lower the ceiling (which will not crush you), and to raise the lift.

The exit itself is guarded by one or two fiends. You can run past them to exit, or try fighting them. If you choose to attack, you may need to defeat upto four fiends and one shambler, depending on difficulty. However, only hard difficulty has them wait patiently for you to approach.

If you somehow fall down the lift after it lowers, or if you lagged behind in a co-op game, a teleporter is available to return you to the exit.

Secrets

 * 1) In the first large room on the water level, there's an underwater passage that leads to a ring of shadows. Reached by jumping down the left-hand side from the upper ledge.
 * 2) When returning from the gold key, there is a shootable door below the ogre on the right.  Contains 25 health and 10 rockets.
 * 3) When approaching the exit, there is a yellow armor behind some bars.  To retract the bars, you need to kill the two ogres in the rafters above.  Once they are dead, the armor is accessible.  To claim the secret, shoot the wall at the end of the alcove and use the teleporter.  Jump immediately to claim the 10 rockets.