Aggersborg is the largest of Denmark's former Viking ring fortress, and one of the largest archaeological sites in Denmark. It is located near Aggersund on the north side of the Limfjord. It consists of a circular rampart surrounded by a ditch. Four main roads arranged in a cross connects the fortress center with the rampart's outer ring. The roads were tunneled under the outer rampart, leaving the circular structure intact. Many archaeological excavations have been conducted on the site, revealing its original structure and design. These excavations also uncovered a large number of artefacts from the Iron Age and Viking Age. The surface of the site as it exists today is a reconstruction.

A Viking ring fortress, Trelleborg-type fortress, or trelleborg, is a type of circular fort of a special design, built in Scandinavia during the Viking Age. These fortresses have a strictly circular shape, with roads and gates pointing in the four cardinal directions. Inside the fort, each quadrant has one, in a single case four, square blocks of longhouses, completing the geometric symmetry. There are a total of five confirmed Viking ring fortresses at present, located in Denmark. They have been dated to the reign of Harold Bluetooth of Denmark, with an estimated near contemporary time of construction c. 980. Their exact historical context is subject to debate. In 2023, the five Danish forts were inscribed on the UNESCO World Heritage List because of their unique architecture and testimony to the military power of the Jelling Dynasty.