It broadly depicts the wars in, and migrations to, the island of Great Britain in the centuries from the Roman invasions to the Norman Conquest.

[2] Britannia has spawned a subgenre of wargames characterized by epic time scales with players taking the part of multiple tribes or nations over the course of the game.

The game begins with one army in each area of Britain, with each part of the island occupied by different nations: the Belgae, Welsh, Brigantes, Caledonians (representing the distinctive Broch culture) and Picts.

Meanwhile, throughout the game, further nations arrive from across the seas: the Irish and Scots, and later the Dubliners, from the west; the Norsemen from the north; the Saxons, Angles, Danes and Norwegians from the east; the Jutes and Normans from the south.

At the end of the game all four players will have the possibility to become the King of England through their control of Harold the Saxon, William of Normandy, Harald Hardrada and Svein Estrithson.

An ideal game ends with a close finish with several players still being able to win, and the outcome resting on one battle or roll of the dice.

A German-language version was released by Welt der Spiele and the game was later republished in the United States by Avalon Hill.

The rules included Isle of Man (part of England) and 4 Irish areas, an extra leader 'Brian Boru' was also an option.