The large number of hill forts throughout pre-Viking Age Sweden and Norway points to a society where conflict was prevalent and a means of defence for individual communities of paramount need.
By the time of the Vikings some of these forts had been superseded by towns, which were sometimes surrounded by walls, banks, palisades or ditches and served as places of refuge for the population of the surrounding areas.
The town of Birka on the island of Björkö in Lake Mälaren, Sweden, was a major trading town and protected by a wall and rock fortress, while Hedeby sat within a narrow fjord with a fort to the north and a semi-circular rampart about it. Grobin in Latvia was surrounded by strong earthworks, while the Danevirke – built along the southern Danish border from 808 on, initially to withstand the armies of the Emperor Charlemagne – protected Hedeby and the nearby Schleswig.
Many fortified camps were constructed by Viking armies when they were camping over winter in occupied territory, with a good number being recorded in England, in particular.
Military camps at Trelleborg, Aggersborg, Fyrkat and Nonnebakken in Denmark were more permanent and impressive in scope. These barracks were surrounded by a turf and timber bank and a ditch, forming a circle with four gates, probably protected by a tower above each. The barracks were arranged in a regular pattern in the four quadrants of the circle: the largest at Aggersborg had 12 in each, while the others had four, all built in groups of four around a square courtyard. Trelleborg also had an additional 15 houses within an outer defence-work. They were undoubtedly built to house a military force, perhaps even the invasion armies of Sven Forkbeard and Cnut.
Another such camp was Jomsborg on the Baltic coast, mentioned in Snorri Sturleson’s 'Jomsvikinga Saga', and associated with Wollin in Poland. The Jomsvikings were a warrior community of mercenaries who lived by a strict code, all being men between the ages of 18 and 50 with no women allowed in camp. They shared all the spoils of war evenly and avenged the death of their fellows. Their fort had a harbour with a stone arch topped by a tower with iron gates, and held a number of warships. Though the tale is fiction the camp is described in surprising detail and may have been founded in truth.