The Sentinel is an extraordinary game

Written in 1986 by Geoff Crammond, The Sentinel is a masterpiece that combines formal beauty and real thrill. Design wise, the game shows that, by properly embracing constraints, one can achieve a highly coherent system.
Screenshots in this post are from Zenith, a rewrite released in 2005 by John Valentine.
Map view of an island

How it works

You wake up on a mountainous island. The place is guarded by the Sentinel, a creature that sits on the highest point and watches the land in a slow 360° rotation. Your goal is to dislodge the Sentinel and take its place.

(Note the link to a demo on YouTube at the end of the post.)

You have limited energy. Whenever the Sentinel detects you in its field of view, it stops rotating and begins to absorb your energy and game’s over when you run out of it. The Sentinel fries you in a few seconds if you’re weak, or that can take half a minute if you’re real strong.

You escape the lethal beam by jumping away, which involves creating a copy of your body and teleporting yourself into it. Creation costs energy so to stay alive and keep moving you must rebalance the energy level by absorbing trees, abandoned bodies, etc.

In advanced levels, the Sentinel is assisted by nasty red guards: the Sentries.

Make a boulder stack to climb faster

Your lifecycle, in a nutshell:

  • Locate a place to jump to
  • Create new body there (energy loss)
  • Transfer yourself to new body
  • Absorb old body (energy gain)
  • Repeat until you can absorb the Sentinel

Problem: You can only absorb an object if you’re high enough, relative to it. You need to see the base tile where it stands. That applies to any object including the Sentinel itself: In order to absorb the latter and win, you must elevate yourself until you can see its own tile from above.

When preparing to jump, thus, the key is to target a tile that’s higher than your current one. Obviously you can’t see ground that’s much higher than you, but you can see ground that is slightly higher. To climb faster, stack a couple of boulders then place a body at the top.

Two nasty sentries, saying their prayers

Atmosphere

It feels like wearing a heavy diving suit, looking desperately for a place to hide, knowing that the shark’s coming.

Despite the abstract representation of the world, the game soon manages to put a huge pressure on you. Mainly because you can only look around quite slowly, with the constant risk of being caught by the Sentinel’s eye. At times you won’t find a good next tile and you start to panic. Synthetic music and sounds add to the stress and bizarreness.

Now let’s talk design.

“It is time… It is time… It is time… For stormy weather!” — Pixies

Design

In 1986 you can compute complex 3D views but can’t really animate. Crammond seems to design the entire game accordingly. He does that with so much genius that the technical limitation becomes virtually invisible.

Geoff Crammond can’t offer animation for two key features where you’d normally expect some, so what he does is twist your expectations.

Here’s how:

1. Travel to next location

He comes up with the teleportation idea to tackle this. Teleportation is instant thus doesn’t require animation, does it?

2. Swing camera around

Continuous panning isn’t an option so Crammond does it by increments: When you want to look left he computes the view from the next sector to the left (this could easily take a second on 8-bit computers!), stitches that new view to the current one and pans the whole. Resulting in a chopped scrolling effect — see demo below. Not ideal.

In order to mitigate the annoyance, Crammond paradoxically seems to fully embrace it.

He renders the effect normal. How? By having the Sentinel rotate sector by sector too. Now all things considered, your camera movement can’t fairly be called choppy: In this world apparently, it’s just the way bodies rotate.

What’s amazing is you barely realise all views are static. By design, the game forces you into a mental model where you simply forget there’s no animation.

(Similar constraints, I suspect, helped shape Toy Story in the early 90's. It’s way easier to match your expectations in terms of realism with plastic toys than it is with human characters.)

The game rules are beautifully simple too, all about energy management and timing. Grab a few more trees or get out of here right now? The interface, graphics and sounds are as minimalist as the rules.

The whole feels incredibly coherent.

The Sentinel on her pedestal and her last Sentry — you’re almost there

Geeky details

After the first land, you’re randomly hyper-spaced to one of the 9999 remaining ones. Since lands are computer-generated, the application binary, once zipped, weighs no more than 24 kilobytes (Amstrad CPC version). You read this correctly. You’ll never get more gameplay per byte!

That’s about 1% the size of this post in PDF.

Nineteen years later, Valentine has fun too:

The only hand-prepared artwork you’ll see is the loading splash screen (1/3 of the size of the game archive, in terms of storage) — Zenith dev blog

A modern remake: Zenith

Zenith remarkably preserves the spirit of the original. Surely the best Sentinel offspring around today, Valentine’s adaptation is a superb homage. It reinforces the timeless elegance of the game while adding fluid animation, nicer colors, fog effects and countless tweaks. Very contemporary.

Yet, while animation improves the usability, the gameplay isn’t significantly enhanced. This confirms the genius of Crammond’s original design:

The Sentinel is a virtual reality game that doesn’t require animation to feel real.

I’ve been in touch with John, we’ve discussed the game via long emails. He made Zenith on his free time and abundantly documented his design and implementation choices. That’s freeware, but donations are welcome.

Zenith is for Windows only but seems to work fine on other systems, using a Windows emulator. See my extra tips especially if you’re on Mac.

Now I dream of a true VR version. Should be pretty insane.

Demo and links

Demo of the original game, The Sentinel, released by Firebird (Atari ST version)