General comments:

Oozes... they're everywhere! But this time you have the power in the form of a MP-41 Submachine Gun. The MP-41 makes short work of those Oozes but watch your ammo count. You'll find a few MP-41 Magazines down here but not enough to replace the ones you use killing all the Oozes. The wide corridors on this level (see map) make a nice change from the cramped conditions on previous levels.

Near the middle of the map you'll find Walter.

Conversations with the Dead:

Read the Conversation with Walter.

As we expected Walter and the other German came down through "Feel The Power" in search of the wooden box rather than upwards from "Happy Happy, Carnage Carnage" through The Labyrinth:

We didn't meet them on the way down, but
they were everywhere when I came back. It
was like everything was asleep and we
slowly woke it up.

Based on what Behrens said on "Wrong Way!" we can assume that Walter and co. split off from the main group while on Ground Floor. As Walter explains they didn't meet any opposition on the way down (unlike us):

The pyramid slowly awoke behind his,
trapping us down here.

This awakening of the pyramid helps to explain a number of points later on.

But back to Walter's mission. Muller gives Walter a gold key and tells him to find some gold ingots in a locked room and also a wooden box:

Muller sent us to get one gold ingot and a wooden
box, he said they were extremely important.
I was leaving though, there was no way I
was going back down to try to find him.

and...

Muller said we needed all twelve ingots, but
that the box would duplicate things which
were placed inside it so we should only get
one. We found the box.

So where did Muller get all this informaton and the gold key itself? Same place as the Alien Pipes?

The Spanish speaking people feature again. It seems they knew what to look for:

I had the key, but someone took it. A group
of people passed by, speaking Spanish, and
their leader took it.

and...

I couldn't understand what he was saying,
but it seemed like he was expecting to find
the key.

All very intriguing.

Items to find:

One Gold Ingot (one point and also worth $38.0K).
One Clear Blue Potion.
Rusted MP-41 Submachine Gun.
Seven MP-41 Magazines (one loaded in the gun).

Points to note:

When you pick up the Gold Ingot your total points will increase to 19, one more point and your health will increase by a further two points. Things are looking up.

Walter's MP-41 is described as "rusted" unlike the others on "Happy Happy, Carnage Carnage". Must be all the Oozes down here. It doesn't work either not that we need a fourth MP-41.

As Walter says the Oozes roam around in groups of three down here... well most of them anyway.

You can forget the bottom half of the map since all the items to find are located in the top half.

Monsters to kill:

Oozes and a few Ghouls.

Decision to make:

You see a crystal ladder going down. Descending to A Plague Of Demons.