THE purpose and function of the Danish geometrical Viking fortresses of c, 980 (the Trelleborg type) has been much disputed Over the years. They are known, from archaeology but not mentioned in a single written source. In order to understand them we must also investigate their context as known from, both archaeological and written sources. This can now be done on a much sounder basis than before, for during the last decade or so Viking research has been very active in Denmark, as in other countries. Much new material has been found, and two of the fortresses and some other large-scale Viking structures have been precisely dated by dendrochronology.

Today four geometrical Viking fortresses are known from Denmark. They are spread over the country: Trelleborg on the island of Sjslland; Nonnebakken on the island of Fyn in the town of Odense; Fyrkat in north-east Jutland; Aggersborg in north Jutland, on the shore of the Limfjord (figs 1–2). There may possibly have been more of them, and it may be noted that the group is known today by chance: the excavation of the first of the fortresses, Trelleborg, was started in 1934 as a trial excavation by the National Museum only because the young motor cyclists from the nearby town of Slagelse wanted a convenient area to exercise their machines. Their eyes had fallen on this ring-work — which they did not get.

Trelleborg (figs 2–3) caused a sensation. Nobody had thought the barbaric Vikings able to plan, organise or construct such a sophisticated structure, and the learned world consequently had to rethink their concept of Vikings.

The country was now searched for parallels, and the first to be identified was Aggersborg (figs 2 and 4). After excavation of a limited area immediately after the second World War the parallel was clear. Aggersborg was only much bigger than Trelleborg, and the excavations went on, with interruptions, till 1952. Next came Fyrkat, excavated in the 1950s, The last to be identified was Nonnebakken, also in the 1950s, Very little was left of that fortress, for here there had been a nunnery in the twelfth century (hence the name), and most of the rampart was removed in 1909,