Jomsborg is freely based on an ancient viking game. The game board is made to symbolise the viking stronghold Jomsborg and the game is centred around the struggle between Harald Bluetooth (Harald Blåtand) and his son Sweyn Forkbeard (Svend Tveskæg). The player playing Sweyn starts at the center of the board with 1 king and 28 vikings. Harald starts as an outer rim on the board with 1 king and 47 vikings. The players take turn moving one piece at a time. Pieces are moved horizontally or vertically - never diagonally - on the board. Vikings can move as many or few spaces as they want in one direction without crossing other pieces. Kings can only move one space at a time. Kings can only be beaten by a king moving to the same space. Vikings can be beaten by having enemy pieces on 2 opposite sides of it. Beaten pieces are removed from play. A piece is captured if it is surrounded by pieces on all 4 sides and at least one of these are an enemy piece. Sweyn wins by having his king reach the outer rim of the board or by capturing or beating Harald. Harald wins by capturing Sweyn or by beating him.