Ringbreak: the Viking board game
Ringbreak is a board game and follows the rules of Copenhagen Hnefatafl, the tournament standard of the old Norse tafl game. Two unequal sides: 24 attackers along the edges against 12 defenders and their king in the middle.
It is a logic duel against the computer — no dice, no hidden material, everything is on the board. Free in the browser and playable without an account.
You play the defenders, and that is a decision rather than something the rules impose: at equal search depth our evaluation wins far more often as the defenders than as the attackers. Offering the choice would mean offering a game already decided. Once the gap is smaller, the choice gets unlocked.
The rules
- Pieces move like rooks. Across or down, as far as you like, never through another piece and never diagonally. The king moves like everyone else.
- The attackers begin. After that, alternately.
- Five squares are special: the four corners and the throne in the middle. Only the king may stand on them, and he may return to the throne. All five act like an enemy piece for their neighbours even while empty — so a capture can be made against an empty throne.
- Captures are made by custody. A piece that ends up between two enemy pieces in a straight line after a move comes off the board. A piece that moves in between two enemies by itself stays — the capture belongs to the mover, not to the shape.
- The king needs four sides. At the edge he therefore cannot be captured; next to the throne, the throne counts as the fourth side.
- The defenders may not repeat a position. The rule names them explicitly: a side that can dig in should not be able to do it forever.
How a game ends
- The king reaches a corner — the defenders win (rule 5).
- The king is captured — the attackers win (rule 6).
- Exit fort — the king stands at the edge in a shelter the attackers cannot open, and the defenders win (rule 5.1). We read that rule strictly: the wall must not only be sealed, it must hold — no attacker move may capture a piece of it.
- Encirclement — the attackers form an unbroken ring around every defender, and they win (rule 6.1).
- Shieldwall — a whole row along the edge can be taken at once if it is bracketed at both ends and capped from outside (rule 3.1).
- A side to move with no legal move loses (rule 7).
The exit fort and the encirclement are not procedures in the rule text but claims about all future moves ("it is impossible for the attackers to break the fort"). How we test them is our decision, written down in SPEC 23.2 — not something read off the source.
Four positions, step by step
Not a game but four positions, each showing one rule. They were picked by hand — but they run through the same engine that plays: every move below is a legal move, and what holds afterwards is not in the text, it comes back from the rule check.
Example 1The capture — The attacker slides across like a rook. The defender then stands between two attackers in a straight line, and that takes it off the board. Only a piece that is caught this way is captured, never one that moves in by itself: a piece may walk between two enemies and stay where it is. The engine's verdict: The game goes on.
Example 2The king is captured — The king needs all four sides, not two. Three were already there and the fourth attacker slides in — and that ends the game. This is the win the attackers play for. The engine's verdict: The attackers win — the king is captured (rule 6).
Example 3The king reaches a corner — The other half: with the king on one of the four corner squares the defenders have won. He moves like any other piece, so as far as he likes in a straight line — four squares at once is one move, not a sprint. The engine's verdict: The defenders win — the king is on a corner (rule 5).
Example 4Walled in at the edge — Here two rules meet without either of them saying so. At the edge the king is not capturable: one of his four sides is off the board, and that side does not count. He can be walled in all the same — and a side to move with no move loses. The engine calls this rule 7, not rule 6. The engine's verdict: The attackers win — the defenders have no move left (rule 7).
Play it yourself
Ringbreak plays against a bot at a fixed search depth — so "medium" is always the same strength and does not depend on how busy the server is. On a phone you drag rather than tap: a square is narrower than a thumb, and a slip should never be a legal move.
Against a person: the match
An invitation link sets up two games against the same person, with the sides swapped: you lead the besiegers once and the defenders once. There is no side to choose, and that is deliberate — Copenhagen 11×11 is not balanced, so anyone allowed to pick would pick the advantage.
The two games run one after the other: the second starts when the first is over. While the match is still going, the first game's result stays sealed — neither the winner nor the number of moves is shown. Both are revealed together at the end. The reason is in the scoring: when both wins fall on the same side, the shorter game decides, and whoever attacks second should not have a number in front of them that they merely need to undercut.
Up to 48 hours per move, so you can play it in one sitting or take days over it. Letting a deadline pass or resigning loses the whole match — including a game you had already won; otherwise resigning would be an escape hatch out of every bad position.
The match is the mode for two people willing to give each other the time: two full games, one after the other. If that is not what you want right now, the bot above is still there, and so is the puzzle duel, where each of you solves on your own.