General comments:

A small level (see map) yet it packs a punch. Tons of Ghasts, Green Oozes (aka Yuck Monsters), and the granddaddy of the them all... the Giant Purple Mutant Hellbeast (aka Barney on Steroids). We'll get to that particular monstrosity later.

The doors on this level have a habit of closing behind you cutting off your retreat. But hey!... who retreats?

Near the start you'll find the body of Juan and close by a discarded AK-47 SABOT magazine. These are armor-piercing rounds for the AK-47 Assault Rifle. Start duplicating these immediately. You can basically forget about all the other ammunition at this stage. SABOT rounds will get you through the rest of the game.

Conversations with the Dead:

Read the Conversation with Juan.

Juan reveals the purpose of their mission:

I was part of a six-man group looking for
some sort of enchanted bottle- I never
really believed it. But we were ambushed
here, and all died.

and...

It was all Javiers idea. He said the bottle
held great power, and that we would find
much gold on the way.

Remember what Friedrich revealed on Happy Happy, Carnage Carnage:

We were searching the pyramid and it's
dungeons for what Muller described to me as
"a small glass vial of immense power."

and...

He wouldn't want me to tell you this, but he
was convinced that it contained the essence
of an imprisoned demon.

Muller wasn't particularly happy when he learnt that we knew about the vial:

No one should have told you about the vial!
Who said that? Friedrich? I'll kill him...

So the Spanish speaking group were also looking for the same glass vial. How interesting.

Juan died of snake poisioning and it must have effected his memory. Note how he claims not to remember the names of his companions:

I think my name is Juan. I can't remember
any of my companions names.

Yet he mentions Javier and Carlos in his conversation without any hesitation.

They killed and then absorbed everyone,
until only their weapons were left. Maybe
Javier and Carlos escaped, but I doubt it. I
ran back here and hid.

Clearly confused.

Juan gives us a clue as to how to deal with the Green Oozes.

These green, humanoid slimes attacked us,
and devoured everyone but myself. For some
reason they completely ignored me, and took
my friends.

Of course it's far from obvious what the clue is and you won't be proceeding any further until you solve it. The Green Oozes are immune to all weapons and crystals and basically as soon as these guys start moving you're dead. Jason Jones offered this advice in a comp.sys.mac.games post back in Dec 1993.

Juan wasn't devoured by the Green Oozes because he was poisoned. Thus you have to poison yourself with a Thick Brown Potion to get past them. You do have one... right? Some helpful tips:

Makes sure you take it before you pass through the doorway leading to the area with the Green Oozes otherwise they'll freeze in the passageways blocking your way.

Make sure you have full health before taking the poison. Once poisoned your health immediately starts to drop and continues until you die. Full health will give you sufficient time to pass through and survive.

As you pass the Green Oozes frozen in their alcoves you'll find two AK-47s lying in the passageways. Juan said we might find these weapons:

They killed and then absorbed everyone,
until only their weapons were left. Maybe
Javier and Carlos escaped, but I doubt it. I
ran back here and hid.

and...

You should be able to find their weapons, if
someone else hasn't taking them. It's
nearby.

Since we find only two guns and there were six men in the group Javier and Carlos must have escaped... right?

Once past the Green Oozes you'll need a Clear Blue Potion to cure yourself. You do have one left... right? Of course once you find out what's behind the final door on this level (see map) you might not want a cure!

Ghasts! They're Everywhere! The last room is chock full of them. Shake and bake time! But wait that's not all... around the corner is... BARNEY! Like the Green Oozes this monster is never actually named in the game. You can't kill it and its death dialog is pretty vague. The Official Hint Book refers to it as the:

Giant Purple Mutant Hellbeast (aka Barney on Steroids)

Here's the description:

No, you can't kill this one either. "It can't be bargained with. It can't be reasoned with. It doesn't feel pity, or remorse, or fear. And it absolutely will not stop... ever... until you are dead." (from The Terminator)

All you can do is run (OK walk!) until you reach the exit... teleporter?. Yup no more ladders from here on in folks... this is the express elevator to hell! Before you take it grab that Lead Box and Pale Violet Potion. When you pick up the Lead Box your total points will increase by two (total points = 36) and your health units will increase from 22 to 24. Very handy!

When you get a quiet moment take a look inside the box! Woohoo... a Ruby Ring. Should you wear it? Remember the bad experience with the Amethyst Ring? Well the Ruby Ring has quite the opposite effect. Wearing it speeds up Crystal recharging! Keep it on... it matches your eyes! ;-)

Oddly the Official Hint Book states that the Ruby Ring is worth 0 points. In addition it makes no reference to the worth of the Lead Box. Yet when we pick up the Lead Box our total points increase by two and treasure value by $38K. Based on the worth of the Amethyst Ring we might conclude that the Ruby Ring is worth one point and $38K in treasure and the Lead Box makes up the additional point. While the Lead Box might not appear useful it would be very wise to keep it. :-)

Items to find:

AK-47 SABOT Magazine (x30)
AK-47 Assault Rifle (with magazine x19) (on Juan)
Three AK-47 Magazines (x30) (on Juan)
Gas Mask (on Juan)
AK-47 Assault Rifle (with magazine (x19)
AK-47 Assault Rifle (with magazine (x6)
Lead Box (worth one point)
Ruby Ring (worth one point and $38K)
Pale Violet Potion

Points to note:

According to Matt Soell (Bungie), in an early draft of the Official Hint Book the Giant Purple Mutant Hellbeast was called "Malice". Matt wrote:

The Big Blue Meanie was named "Greed" in the draft. Likewise, the "Giant Purple Mutant Hellbeast or Barney on Steroids" was originally named "Malice" and the "Flaming Smokey Dude" went by the name "Deceit." Jason does not remember if these names were his idea or not, nor does he remember specifically why they were dropped in favor of the more colloquial descriptive names found in the final hint book.

Monsters to kill (or not as the case may be):

Ghasts, Green Oozes, and the Giant Purple Mutant Hellbeast.

Decision to make:

You see a teleporter. Teleporting to Don't Get Poisoned.