The Sentinel: The Strangest Game on the C64

Every game has relatives. You can put almost anything on the family tree if you’re patient enough: this shooter descends from that shooter, this open world learned its map from that one, and the whole business turns out to be a few dozen ideas being handed down and argued with. The pleasure of the job is finding the ancestor.

Then there’s The Sentinel, which sits on the tree with nothing above it and almost nothing below.

Geoff Crammond wrote it for the BBC Micro and Firebird published it in 1986, with conversions following to the Commodore 64, the Spectrum, the Amstrad and later the 16-bit machines. It is a first-person game with no gun. It is a puzzle game with no puzzle pieces. It is a strategy game where the only enemy is a stationary figure on a plinth who does nothing except turn slowly on the spot and look at you. It sold respectably, won everything going, and then the industry looked at it, admired it, and quietly declined to make another one.

The landscape as the mechanic

The first thing that hits you is the geometry, and on a C64 it hits you slowly.

The game generates a landscape out of flat-shaded polygons — solid 3D, filled surfaces, on an 8-bit machine whose usual answer to three dimensions was wireframe and hope. The terrain is a chequered grid of squares at varying heights, some in shadow, some lit, receding to a horizon. Drawing it takes the C64 a good long moment; you sit and watch the world assemble itself in strips while the drive stops clicking. Once it’s up, turning your view is a series of discrete lurches. This is not a machine pretending to be smooth.

That slowness turns out to be structural. The Sentinel is a game about looking, and looking carefully, and a frame rate that punishes idle head-swivelling makes you look with intent. You scan the terrain the way you’d scan a chessboard: what’s high, what’s low, what can see me, what can I see.

Because sight is the entire rule set. You can only interact with a square if you can see it — genuinely see it, with the game doing the visibility maths on the polygons in front of you. Height is the currency of sight. A square lower than you is safe to be seen from; a square higher than you is a threat, because whatever stands on it looks down into yours.

There are ten thousand of these landscapes. Crammond didn’t author them; he seeded them, generating each one from its number, 0000 through 9999, which doubles as the password you’re given on completion. It’s the same trick David Braben and Ian Bell had pulled two years earlier — Elite fitted eight galaxies into 32K by shipping the instructions instead of the data — and it’s arguably the more interesting application, because Elite’s galaxies only had to be plausible, while Crammond’s landscapes had to be solvable. Ten thousand procedurally generated puzzles that all have an answer is a much harder promise to keep than two thousand planets that all have a name.

Absorb, create, teleport

You are a Synthoid. You arrive on the lowest square of the landscape with ten units of energy. Somewhere above you, on the highest plinth in the world, stands the Sentinel. You have to get up there and absorb it. That’s the whole game.

The verbs are absorb and create, and they cost the same. A tree is worth one unit. A boulder is two. A Synthoid is three. Absorb a tree you can see and you gain a unit; create a boulder on a square you can see and you lose two. Nothing is destroyed and nothing is conjured — the total energy on the landscape is a fixed quantity being shuffled between forms, including the form that is you.

The movement is where it gets strange, and where it stops resembling anything else. You never walk. You climb by creating a Synthoid — a spare body — on a higher square you can see, transferring your consciousness into it, turning round, and absorbing the body you just left. Three units to make it, three units back when you eat your own corpse. The net cost of ascending is zero, and you have just committed something the game is entirely unbothered by. This is a game whose core traversal mechanic is serial suicide, and it never once comments on it.

Where do the spare units come from? The landscape’s trees and boulders, and eventually the Sentries: subordinate figures the Sentinel posts around the terrain, each one rotating on its own plinth, each worth three when you finally get above it and take it. Stack a boulder on a boulder to stand higher, put a Synthoid on top of the stack, hop into it, absorb the tower behind you, look for the next vantage. You’re building your own staircase out of the world and eating it behind you as you go.

An enemy that only rotates

The Sentinel does one thing. It turns.

Its gaze sweeps the landscape at a fixed, unhurried rate, and if it can see any part of the square you’re standing on, it starts taking energy off you — one unit at a time, converting each stolen unit into a tree somewhere else on the terrain, keeping the books balanced. Run out of energy while it’s looking at you and the game ends.

There is no chase. There is no attack animation. There is a slow figure on a hill, turning, and the mounting knowledge that in about nine seconds it will be facing your way and you need to be standing somewhere it can’t see. The horror is procedural and entirely geometric. Nothing in the game is hostile in the way a sprite is hostile; the hostility is in the sightlines.

The Sentinel can also produce a Meanie — a rotating tree that, when a Sentry’s beam crosses it, hyperspaces you to a random square. Hyperspace is also your own escape hatch, at a cost of three units, and it dumps you wherever it dumps you. You can spend an entire landscape carefully surveying an approach and then be flung to the far side of it, standing in the open, on a low square, with four units left.

The endgame is a small procedural masterpiece. You get above the Sentinel’s plinth, absorb it, absorb the boulders it was standing on, place a Synthoid on the now-empty summit, step into it — and hyperspace out from the highest point in the world, which is what the game meant by winning all along. You leave by taking the throne and then leaving the throne.

Why nobody copied it

The obvious answer is that it’s hard to sell. There’s no shooting, the frame rate is a series of considered opinions, and explaining the appeal takes four paragraphs.

The better answer is that The Sentinel solved a problem nobody else had. Crammond came to it from Aviator and Super Invaders on the BBC, and went on to Stunt Car Racer and Formula One Grand Prix — simulations with the maths sanded until they feel like driving. Genre iteration was never the habit. The Sentinel is what happens when that same instinct for a clean rule set gets pointed at a landscape instead of a car. It’s one mechanic — visibility — taken absolutely seriously and given a world to operate in.

Games that get made in numbers are games with a shape other developers can borrow. Boulder Dash gave everyone a physics grid they could reuse; Thrust gave everyone inertia and a tether. The Sentinel’s shape is unborrowable, because there’s nothing to strip out. Take away the height-based sight and you have nothing. Add a weapon and you have nothing. It’s a single load-bearing idea with the entire building on top of it.

Crammond returned once, sort of: Psygnosis published Sentinel Returns in 1998, with a John Carpenter score and modern polygons, and it is a faithful, competent, slightly airless remake that proves the point. The original’s ugliness was doing work. The long draw time, the lurching turns, the flat untextured squares — they force the pace that the puzzle needs. Make it fluid and you’ve made it possible to fidget, and fidgeting is death in a game about deciding where to look.

Where it stands now

The C64 version is the one I met it on, and it’s the compromised one: slower than the BBC original, drawing its world in visible instalments, and better for it in ways nobody intended. The 16-bit conversions are faster and cleaner. Emulation makes all of them trivially available now, and there are faithful open reimplementations knocking about for people who want it on a modern screen. Any of them will do — the game is made of rules, and the rules survive the port.

What doesn’t survive is the context. In 1986 a solid-3D world on a breadbin was somewhere between a rumour and a magic trick; the technical achievement was half the sell, and the C64 chugging out its polygons was the point of the demonstration. Strip that away and you’re left with the design, which is the part I’d argue has aged better than almost anything else from the decade. Ten thousand landscapes. One enemy. It looks at you, and that’s the threat.

Forty years of games have found other things to descend from. The Sentinel is still standing on its plinth, turning slowly, with nobody around.