The Sentinel is a strategy game written by Geoff Crammond in 1986. It is a masterpiece that demonstrates that with the right design, it's possible to turn a technology constraint to your advantage and deliver something brilliant.
Note: screenshots used here are from Zenith, the 2009 remake of the game (see below)
Game rules
You wake up on a mountainous island, represented in 3D. You're not alone, there's the Sentinel guarding the land. The Sentinel is a creature located on the highest point of the island; it continuously scans the whole land in a slow rotation, a bit like a lighthouse. Your goal is to "shoot" the Sentinel, then take its place.Problem: whenever it finds you in its field of view, the Sentinel starts pumping your energy. When you have no energy left, the game is over. So you constantly need to escape from the Sentinel's stare. Why not shoot the Sentinel as soon as the game starts? Because what you need to shoot is not the Sentinel itself, but the base it stands on. This assumes you see it from above, which requires that you are located a bit higher than the Sentinel; when the game starts, you stand at a low point.
How to move? Actually you can't really. You are a kind statue or robot fixed to the ground, or more exactly you wear the robot. Your only way to move is by teleporting yourself from your current robot shell to a new one, that you can create remotely. When not moving, you can still rotate - as the Sentinel does - to look around for objects to absorb, their energy being required for you to create the robot shells.
So your life cycle is simple: find a place to move to, create a robot shell there, jump to it, absorb objects around to rebalance your energy capital, find a new place a bit higher, create a robot, move to it, and so on, until you can climb high enough to absorb the Sentinel.Atmosphere
Despite the very abstract representation of the world, the game soon manages to put a huge pressure on you. This is mainly because you can only move slowly while constantly at risk of being caught by the Sentinel's eye. Sometimes you won't be able to find a place where to jump next, and start to panic! The feeling is like wearing a heavy diving suit and painfully looking around for a place to hide into, knowing that the sharks are coming.About the design
Keep in mind it's 1986 and the basic home computers The Sentinel is meant to run on are not able to animate plain 3D vectorial scenes easily. So 3D views can be generated, but not well animated. Crammond leverages this constraint and designs the entire game based on it. He seems to come up with two key ideas, that helps him overcome the impossibility of complex animation:
- As a concept, the use of instant teleportation to justify the absence of animation during your transfer from a robot shell to another;
- As a technical trick, a partial refresh of the scene when the camera swings around. The camera rotation is not continuous but is done by bits, which allows the program to simply scroll a series of static images computed just-in-time. (It's a little more complicated than that but basically this is the idea.)
Crammond can't use animation, therefore he invents a world where it's natural that bodies don't slide from A to B, but disappear from A and re-appear at B. What's amazing is that you barely notice all views are static; this is because in this world, you simply don't expect things to move. Similarly, it's not confusing to see the camera moving by bits rather than in a continuous motion: look at the Sentinel, she too scans the land sector by sector. In this world, that's the way objects rotate.
The rules of the game themselves are beautifully designed, all based on the simple principle of energy management (absorb/spend). You can absorb any entity provided you can see the square it's on, and this applies to trees, the Sentinel itself and even your abandoned robot shells. The graphics and the interface are elegant and minimalist, just like the rules.
Once you win the first land, you are randomly hyperspaced to one of the 9999 remaining ones... Lands are computer generated rather than explicitly defined, and as a result the whole application can be stored in a file that, once compressed, weighs no more than 24 kilobytes (Amstrad CPC version), which means about 5% the size of this blog post. You'll never get more value per byte!
Playing The Sentinel today
To try the game today your best option is definitely Zenith (2009), the rewrite for Windows by John Valentine that remarkably preserves the spirit of the game. Valentine's adaptation is superb; it sublimates the timeless elegance of the original, essentially by keeping the plain graphic style. Zenith adds realtime animation of the camera, and many subtle details that make it look very contemporary.
Yet, it's interesting to note that, while animation improves the usability, the overall gameplay is not significantly enhanced. This confirms the genius of Crammond's original design: The Sentinel is a virtual reality game that does not require animation to feel real.
Above: The original Sentinel for Commodore 64 and Amstrad CPC, by Geoff Crammond.
See also:
Wikipedia article about The Sentinel
Zenith website
Case study: An overview of Zenith, by John Valentine (about game design)
The Sentinel demo (YouTube) - Atari ST version
A good description of the game by A. Manwaring
An interview with Geoff Crammond
KotH, for iPhone
