Viking Fortresses are a special type of fortresses, built in the time of the Vikings.
Viking fortresses are also called "trelleborge" (trelle-fortresses), which can be confusing, as the name is not just used as a category-name, but also as the name for specific fortresses. A viking fortress near the city of Slagelse on Sealand is called Trelleborg, and another fortress in Sweden is also named Trelleborg, as is the city in which it is located.
6 viking fortresses have been found: Aggersborg by the Limfjord, Fyrkat near Hobro, Nonnebakken in Odense, Trelleborg near Slagelse, Trelleborg in Trelleborg and Borgeby near the city of Lund, both in Sweden.
During the reign of Harald Bluetooth in the 10th century, 4 or 5 viking fortresses were built.
The viking fortresses are laid out as ringforts, with a circle as the common shape. In the circle there were gates against all four corners of the world, and two main streets intersected in the center. Houses were built inside the circle, and we know some of those buildings were used for craft production.
As to what specific reason the viking fortresses were built, speculations abound. There is a consensus that a strong royal power was behind their construction. In all propability they were built for defensive purposes.
The viking fortresses were unearthed in the 20th century, and forever altered the perception of the vikings capabilities and achievements.