Prisma Duel is a free, top-down, turn-based space combat game that runs in a browser. You fly one fighter in a mirrored arena strewn with dichroic crystals. Nothing happens in real time: each turn you set a course, decide whether to fire, and commit — then every pilot's orders resolve together over about four seconds, and you watch what your guesses actually did.

The turn

Everybody plans at the same time, against a clock. When the last pilot commits — or the clock runs out, in which case your fighter simply holds its last course — the turn executes for everyone at once. There is no initiative order and no waiting your turn. The tension is that you are aiming at where someone will be in four seconds, not at where they are now.

One control, and it is out on the arena

Drag the handle at the end of your course line. The direction you drag is your heading; how far you drag is your speed. The arrow keys do the same job if you prefer them, with Shift for a finer step when you are threading a shot through a crystal.

Speed is bought from the same power the laser capacitor wants. A long drag is a fast run with nothing left to charge with; a short one is a tight turn that fills the capacitor. The orders panel shows you the split, but nothing in that panel is settable — the drag is the whole interface. Your course carries over between turns, so holding a curve is what happens if you do nothing.

The laser

The beam is welded to your nose. There is no turret, no lead indicator and no lock. Damage is charged per millisecond of dwell: a graze costs a few points, and holding somebody in the beam for a whole turn is fatal. Because hitting anything means turning to track it, and turning is exactly what your throttle just made expensive, every shot is an argument with your own speed.

Firing spends the entire capacitor and suppresses shield regeneration for that turn. Shields absorb before hull, and they only come back on turns you keep the laser cold.

Crystals, mirrors and the closing walls

Every crystal is cut differently — five to eight faces, each vertex at its own radius and angle, generated from the arena seed — so no two arenas look alike. They are solid cover, and they refract. Fire through one and your beam bends, disperses into a rainbow fan, and can strike pilots who were never in your line of sight. The arena walls are mirrors, so a bank shot is a real tactic, including the bank shot that comes back and hits you. Flying into either is fatal.

From turn seven the box tightens. Once it is as small as it goes it begins to collapse, hurting everyone still alive by an escalating amount, so two careful pilots cannot circle each other forever.

Playing it

Practice puts you against three bots and needs no server at all. Multiplayer takes up to four pilots: enter a room code, or leave it blank to be matched. Any pair of players that can reach each other plays peer-to-peer, and any pair that cannot falls back to a relay, so a match survives most networks.

Drag / Arrows

set your course — Shift for a fine step

F

arm the laser

Enter

commit your orders

0

reset the view

H

hide the interface

?

this page

Where this game comes from: Critical Mass

Prisma Duel is an homage to Critical Mass, the turn-based space combat game written by the British programmer Sean O'Connor and released for Windows in early 1996. Its forerunner was an Atari ST game he worked on between 1988 and 1993, and it was the most ambitious thing he had built at the time — a lot of the work went into the AI rather than the picture, on the reasoning that space is mostly black anyway.

You command a squadron of ships seen from directly overhead on a plain black field, with minimal, almost vector-like graphics. You move each ship by dragging an arrow out from its nose, and how far you can pull it depends on the ship, because acceleration is limited — then you end the turn and everything moves together. The weapons are six kinds of missile that home on the nearest target, which is why so many Critical Mass stories end with somebody killing a friend. A ship that comes apart ejects its pilot, and the escape pod is a little circle that can itself be shot. There are forty mission types: escort a convoy, attack a starbase, defend Earth.

It went out as shareware, in the try-before-you-buy economy of the mid-nineties — magazine cover disks, download sites — next to O'Connor's other games, Slay, Conquest and Firefight. PC Review called it "more playable than many multi-million dollar productions" in January 1996. Version 3.0 arrived in 2001 with better-looking ships and a ship and mission designer, which is the version most people who played it around the turn of the millennium actually remember, and it was rewritten again in 2004. It is still sold and still maintained today, for Windows, iPhone, iPad, Android and Kindle.

Critical Mass at Sean O'Connor's Windows Games →
The full story of Critical Mass, and where to get it →

What is kept, and what is not

Kept: the overhead view, the secret orders, the simultaneous resolution you sit and watch, and above all the control — a course you drag out in front of the ship, with momentum you have to plan around instead of a joystick you react with. That gesture is Critical Mass's, and nothing else has ever felt quite like it.

Changed: one fighter each instead of a squadron; a real spectral ray tracer instead of homing missiles, so the weapon is light that genuinely refracts and disperses through the terrain; and lockstep peer-to-peer play in a browser tab, which was not something you could do in 1996.

If you found this while searching for Critical Mass: go and get the real thing. It is still there, it is still maintained, and it is still better at being itself. This is a love letter, not a replacement.

Read more

How the laser works → the spectral ray tracer, with live examples of Snell, Fresnel and dispersion.
How multiplayer works → room codes, peer-to-peer, and why the match survives the server.
Games like Critical Mass → the rest of the genre, old and new.

Prisma Duel is an independent tribute. It is not affiliated with, endorsed by or connected to Sean O'Connor or Windows Games Ltd.