The Primary Excavation Site houses the ancient burial chamber.
The discovery of this chamber unearthed the stone tablet, soul cube, and many
other valuable artefacts.
You start off in a room with a ton of gear in it:
15 boxes of shells
2 plasma guns
10 plasma canisters
7 ammo belts
3 packs of grenades
4 machine guns
3 rocket launchers
3 packs of rockets
2 suits of 50 armor
2 health packs (30 each)
Gear up and lock and load.
Head into the hallway. The path to the right looks like a dead end, but if you
work your way over the wreckage, you'll find a BFG cell. Go back to the left and
down the stairs. You'll see lots of construction and scientific equipment as you
go. Eventually, you'll work your way to a room with a big coffin in it. A
presentation will be playing talking about the ancient civilization. There are 2
health packs (15 each) on the desk near where the presentation is playing. Take
them, and walk around to the back side of the coffin/sarcophagus. The wall will
disintegrate and leave a door. You'll pick up a backpack (ammo) as you wal
through. Turn right and follow the hall. Another wall will open up and you will
walk into a square room. The floor will start to fall and will evetually stop,
presenting another way out. Follow the path downward. You will evetually get to
a long path downward that had 4 candles at the end of it (3 on one ledge, 1 on
DON'T TURN RIGHT if you want to go to the secret room. Instead, turn left and go
to the back of the alcove. Crouch down and you'll see a stone that says "id" on
it. Press it (you won't see a cursor) and a wall will open to your left. Walk
through and pick up the PDA on the altar. It's for "id software" and has a lot
of cool stuff in it. When you're ready to face the final boss, go back and turn
RIGHT this time.
The final boss is the Cyberdemon. As the soulcube explained the first time you
visited hell, it is the only way to defeat him. The strategy is simple. Kill
normal demons until you get a soulcube shot. Use the soulcube on the Cyberdemon
and repeat. You must hit the Cyberdemon with the soulcube 4 times. Demons spawn
around the pit in the middle, so arm the shotgun (or whatever favorite weapon
you have - I suppose the rocket launcher would be fun) and start running around
the pit in the middle killing demons. Obviously, you need to avoid the rockets
from the Cyberdemon. When you get your soulcube shot (once every 5 kills), be
sure to hit the Cyberdemon - if a normal demon is between you and the cyberdemon,
you may hit it instead, so be careful.
When you shoot the fourth time, the ending cinematic will play. Sequel, anyone?
Congratulations, you've beaten Doom III! Now for nightmare...