Hnefatafl, also known as The Viking Game, The King's Table or simply Tafl, is one of the rare breed of games with two unequal sides. The defending side comprises twelve soldiers and a king, who start the game in a cross formation in the center of the board. Their objective is for the king to escape by reaching any of the four corner squares (haven tile). The attackers comprise 24 soldiers positioned in four groups of 6 around the perimeter of the board. All pieces move like the Rook in chess and pieces are taken by "sandwiching" i.e. moving your piece so that an opponent's piece is trapped horizontally or vertically between two of yours or a refugee tile.
The Attackers move first. The Attackers win by capturing the King.
This team wins if the king “escapes” to one of the haven tiles.
Each turn a player may move one piece any number of tiles in a single orthogonal line. The center tile and the four corner tiles are reserved for the King and no other piece may move there, although the King's (center) tile may be crossed by any piece.
An Attacker or Defender piece that is blocked on two or more orthogonal sides by an opponent’s pieces is captured and removed from the board. A piece can also be captured if it is blocked on one side by an opponent’s piece and on the other side by a haven tile.
A handy quickstart guide in PDF form may be found here.