All material below quoted from: Pulsiano, Phillip and Kirsten Wolf, eds. 1993. Medieval Scandinavian: An encyclopedia. Garland Reference Library of the Humanities; vol. 934. New York: Garland, 216.
The designation (Trelleborg fortresses) refers to four geometrically planned fortresses in Denmark (see map to the right):
They were all built around 980, of timber, turf, and earth, and they lasted for only a short time. They were never repaired.
The Trelleborg fortresses differed in size and in various details:
But they had the same strict overall plan, for which no exact parallel is know: a circular rampart with gates at the four points of the compass; the inner area divided into four streets between the gates; large bowsided houses arranged in regular quadrangles. A full scale reconstruction of such a house was built at Trelleborg in 1948 and at Fyrkat in 1985. The cemeteries of these fortresses have been found outside the ramparts. The Trelleborg fortresses are not mentioned in written sources, but the fact that they were constructed at the same time and at widespread locations in Denmark indicates that they must have been built by the king, most likely Harald (Bluetooth) Gormsson, known for many other achievements and large building enterprises (e.g., Danevirke and Jelling).
The Trelleborg fortresses are the oldest known royal fortresses in Scandinavia, and their main purpose was probably to control Denmark. They were all situated on important inland roads and either had no access to or were a long way from the open sea. Consequently, the theory has been dismissed that they were inter barracks and training camps for the Vikings, who raided and finally conquered England under Harald’s son Sven (Forkbeard) Haraldsson. Chronological reasons also argue against the theory. Perhaps the Trelleborg fortresses were abandoned after Sven’s successful revolt against his father around 986.