You first meet the Sentinel on Lasciate Ogne Speranza, Voi Ch'Intrate but you won't find out what it's called until you kill one with the Green Crystal from Watch Your Step.

Back in June 27, 1999 Abhaya Hess <kanrah@compuserve.com> pointed out that the Sentinel looked like a Zombie or Ghast. They wear the same headdress and robe suggesting that the Sentinel is a living version of the Zombie or Ghast. The result of a possible earlier plot line?

If you look closely at page 12 of the Pathways Into Darkness manual you'll see a small screenshot of the Message Window which includes the line:

You see a dead Monk.

This screenshot is obviously taken from an earlier version of the game but was the Monk renamed the Sentinel? The Sentinel does bear an uncanny resemblance to a monk and indeed the Pathway Into Darkness Hint book colloquially refers to it as the "Skinny Buddha Dude". Buddha like statues do appear in the Yucatan so perhaps Bungie were basing their Monk character on a known past figure.

Note also in the Message Window screenshot that the Yellow Crystal is readied after the dead Monk is seen:

You see a dead Monk.
Yellow Crystal ready.

Why ready the Yellow Crystal unless you are going to begin a Conversation. Was it the case that you could talk to Monks in an earlier version of the game?

It has been suggested that the unseen death dialog:

Seven men in overcoats jump from the shadows and
carry you away. You are never heard from again.

once belonged to the Sentinel. However as yet no conclusive evidence exists to link the two together. The Sentinel's forcefield is harmless but that might not have always been the case. Had the forcefield caused damage the odd death dialog (above) might be construed as appropriate given that you would either have to be very unlucky or rather stupid to get it! ;-)

Tim Branin <Sonofhydra@aol.com> points out that it is not possible to determine if the Yellow Crystal crystal was actually used on the dead Monk since any amount of time can pass between any two events listed in the Message Window.

Tim also goes on to say:

I have two theories about the Monk.

1. Maybe the Sentinel was a living version of a Zombie (red tunic blue headdress), and the Monk was to be a living version of the Ghast (blue tunic red headdress)

2. The Monk may have been part of another sub plot in which missionaries came to the pyramid

Thomas Resnick <tommyr@darkforces.com> writes concerning the "Monk" mystery:

Is it just me, or does it seem like the Zombies, Ghasts, and Sentinels are all part of some odd, arcane cult that has taken up residence inside the alien pyramid? All the other creatures (headless, nightmare, etc) seem to be weird, alien animals, but ANIMALS. These three seem to be a human or humanoid form. The sentinels are sustained somehow, and the Zombies and Ghasts appear to be undead sentinels or something. I keep getting fleeting visions of a cult that centers around the pyramid, with the most fervent members dying and becoming either a Zombie or a Ghast. Maybe this idea might shed a little understanding on the origins of the whole "monk" mystery as well.

On Watch Your Step we find Greg, the last of your Special Forces team and the guy with the bomb, dead behind a Sentinel's forcefield. Greg has no Green Crystal. So how did he get past the Sentinel? This interesting logic puzzle was first raised by Dave Mayer on comp.sys.mac.games back in February 1994. At the time Dave wrote:

From: dwmayer@ouray.denver.colorado.edu (Dave Mayer)
Newsgroups: comp.sys.mac.games
Subject: PiD logic flaw?
Message-ID: <10402@copper.Denver.Colorado.EDU>
Date: 14 Feb 94 20:25:29 GMT
References: <chKh_Cy00iV9EB4FgC@andrew.cmu.edu> <CL6rF9.HHA@usenet.ucs.indiana.edu>
Sender: netnews@copper.denver.colorado.edu
Organization: University of Colorado at Denver
Lines: 20


So imagine my surprise when, on reaching Greg (no spoilers...), I realize that 
he's located BEHIND A DOOR GUARDED BY A SENTINEL! Now, I'd been futilely dashing
around Watch Your Step, using the following logic:

        1. Greg ran through Need a Light? and the only way out is up.
        2. Sentinels can't be killed w/o the green crystal.
        3. I have the green crystal; therefore
        4. Greg must still be outside the Sentinel doors somewhere.

When I found him, I initially thought, "maybe there are two green crystals,"
only to search him & find nothing! HOW DID HE GET THERE?

I assuaged my cynicism by hypothesizing that the dreaming god was trying to
defend himself by sealing off the nuclear bomb. But in that case, why is the
green crystal just lying around undefended? Not much of a scheme, on the part
of ol' whats-his-name.

Any ideas on this? It seems like a gap in an otherwise pretty flawless game.

Dave Mayer (dwmayer@ouray.denver.colorado.edu)

Jeffrey Hoover in a follow-up post came up this interesting explanation:

From: jjh@cisco.com (Jeffrey Hoover)
Newsgroups: comp.sys.mac.games
Subject: Re: PiD logic flaw?
Date: 18 Feb 1994 19:23:48 GMT
Organization: Cisco Systems, Inc.
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In article <10402@copper.Denver.Colorado.EDU>
dwmayer@ouray.denver.colorado.edu (Dave Mayer) writes:

> So imagine my surprise when, on reaching Greg (no spoilers...), I realize that 
> he's located BEHIND A DOOR GUARDED BY A SENTINEL! Now, I'd been futilely dashing
> around Watch Your Step, using the following logic:
> 
>         1. Greg ran through Need a Light? and the only way out is up.
>         2. Sentinels can't be killed w/o the green crystal.
>         3. I have the green crystal; therefore
>         4. Greg must still be outside the Sentinel doors somewhere.
> 
> When I found him, I initially thought, "maybe there are two green crystals,"
> only to search him & find nothing! HOW DID HE GET THERE?
> 
> I assuaged my cynicism by hypothesizing that the dreaming god was trying to
> defend himself by sealing off the nuclear bomb. But in that case, why is the
> green crystal just lying around undefended? Not much of a scheme, on the part
> of ol' whats-his-name.
> 
> Any ideas on this? It seems like a gap in an otherwise pretty flawless game.
> 
> Dave Mayer (dwmayer@ouray.denver.colorado.edu)

Dave-

You're forgetting that the sentinels are living beings. Just because
they don't move while *we* see them, does not mean that they *can't*
move. :-)

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Jeff Hoover            Cisco Systems, Inc.         v-  415.324.5157
MacDude ;-}            jjh@cisco.com               f-  415.688.4575

Now eight years on... same puzzle... here are some of the responses:

Ashley Ling <maikosakura2001@hotmail.com> writes:

Okay this may not seem that logical. But maybe after Greg shot his way through, the Sentinels realized (assuming they can) an attack was under way and sat down to create their forcefield.

Note that some of the Spanish speakers got even farther without a green crystal (or am i mistaken? it's been a while...)

Raul "REB" Bonilla <biomeca@hotmail.com> writes:

On Feel The Power you wrote:

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.

So maybe there was no sentinel blocking the way when Greg reached Watch Your Step, but by the time Sgt. Eddings gets down there, that part of the pyramid is fully awake.

7revor Smith <joemahma7@home.com> writes:

None of the dead people we talk to ever mention the sentinals, right? Maybe the sentinals were "created" by the dreaming god to try and stop the player, after Greg came through there. And, since Greg was killed by skitters, we could assume that the two skitters stuck in there with him were the ones that killed him.

Jeff Draney <ddraney@mac.com> writes concerning the number seven and the Sentinel:

...there are guys that sit in your way and make a force field. There are exactly seven of these guys in the game. Also, while it is impossible to be killed by one, in the resource fork, the string says "Seven men in overcoats jump from the shadows and carry you away. You are never heard from again."

Could the unused death dialog "Seven men in overcoats jump from the shadows and carry you away. You are never heard from again." have been associated with The Sentinel at one stage? xcalibur in a Story forum post writes:

...I believe this text is associated with the Sentinels (who can't attack, so you'll never see this in normal gameplay). It makes sense, since everyone else is accounted for, including the Stalker. There's also the placement of this message in the scripts right between skitters and ghasts, which is around the time you meet the Sentinels.

It has been a commonly held belief for some time that there were seven Sentinels in the game. Some have suggested that this is linked to the unused death dialog "Seven men in overcoats... " However nothing could be further from the truth. Hopper provides the correct number in this forum post. He writes:

I count nine total, five required:

4 on Lasciate Ogne Speranza, Voi Ch'Intrate
- 2 in your path
- 1 guarding a save rune
- 1 guarding a Clear Blue Potion

2 on Watch Your Step
- 1 guarding Greg and the bomb
- 1 guarding a save rune

3 on I'd Rather Be Surfing
- 2 in your path
- 1 guarding a save rune

So nine in total and five need to be overcome in order to progress.