A siege is three jobs on a shared clock, and you are always doing one of them.

The battlefield

One board, divided by water. The ground on your side is yours to build on and theirs is not — you can shell across the water, but you can never place a wall on it. Several castles stand on each bank, bare, with no walls anywhere.

Water seals exactly like wall. A coastline counts as a closed edge, which makes where your castles sit relative to the shore the first real decision you make — and turns the river between you into a defensive asset you never have to build.

The shape of a match

A match opens with Nomination and one Cannon Phase, then repeats a three-phase round until somebody fails to seal, or until the round cap.

Both of you are inside the same phase at the same second. There are no turns and there is no pause — leaving does not stop your match, it just stops you playing it.

The first thing you do in a round is fire, not build. You shell them with what you already have, then repair what they did to you.

The Build Phase gets shorter every round and the pieces get more awkward, so a match that reaches the last round is played under 30% less clock than the first.

Nomination — pick your home castle

You have 10 seconds to choose one of the castles on your bank as your Home Castle. It pays two cannons a round while you hold it sealed, against one for every other castle you seal — so the choice is worth thinking about, and the shoreline is the thing to think about.

You are then given a starting wall around it and an opening cannon grant, and the first Battle Phase begins.

Battle — fire

You aim at a tile and fire. You never pick which cannon shoots: the game fires the next eligible one in the order you placed them, skipping any that is destroyed, cut off, silenced, or already has a ball in the air. It skips — it never waits.

Your oldest guns are your most reliable, and both your rate of fire and your firing positions were decided rounds ago, when you chose where to put them.

Shots take time to fly, and further shots take longer. Everything about a shot is settled the instant you fire it — where it lands and when — so you are aiming at where a wall will be, and a wall built after you fired can still be destroyed by that shot.

A landing shell destroys wall in a five-tile plus: the tile it hits and the four beside it. It lands wherever it falls — on open ground, on rubble, on your own bank if you misjudge it. Castles are immune. Nothing destroys a castle, ever.

At the timer the phase reaches cease-fire: no new shots are accepted, but everything already in the air still lands. Damage never falls outside the phase that caused it.

Build — seal

The board turns to daylight and pieces start arriving: one polyomino at a time, from a shuffled bag, with no queue and no lookahead. You place it or you hold it, and the next one only comes when the last one is down.

A piece can go anywhere on your own ground where every tile it covers is land and is clear of wall, castle, cannon and rubble. Nothing else blocks it — it does not have to touch anything you already own. A ring built in open ground is a legal wall, which is why a summer home somewhere quieter, a decoy wall drawing fire, and a detached battery of guns are all real things to do.

A castle counts as sealed when the outside cannot reach it. The check floods inward from every edge of the board, moving only up, down, left and right — never diagonally — and is stopped by wall, water, castle and cannon. Any land the flood cannot reach is yours.

When the timer runs out you get 4 seconds of grace, and then the phase ends and everything is counted at once: who sealed what, who scores, and who is still in the match.

Cannon — arm

Sealing pays guns: two for your Home Castle and one for every other castle you closed this round. The award is re-earned every round and never accumulates — hold only your Home Castle and you get two a round, forever.

Each gun is 2 × 2 tiles and must sit entirely inside the territory you just sealed. Once placed, it never moves. Anything you have no room for when the phase closes is forfeited — which is the ordinary price of walling tight, not a mistake.

A gun left outside your territory after a later round is cut off: it does not fire, but it is not lost. Wall around it again and it comes back.

How it ends

Failing to seal is the only way to lose. End a Build Phase with no enclosed castle while your opponent has one, and you are out — immediately and finally. No lives and no continues. Your walls stay standing and go inert.

If neither of you is eliminated by the seventh round, the higher score wins. You are told the last round is the last round as its Battle Phase opens, which changes how it is played: if you are ahead, you only have to survive it.

And because nothing is ever owned, being shelled back to one castle is not the end of anything. Territory is recomputed from scratch every round, so the whole board is winnable back on the merits.