Trelleborg, near Slagelse in west Zealand, was the first ring fortress to be excavated - between 1934 and 1942. This lent the popular name 'Trelleborg-type' fortresses to the typology. Subsequently, Aggersborg, Fyrkat and Nonnebakken were (re)discovered, with more extensive excavations continuing in Aggersborg (1945-1952) and Fyrkat until the mid-1960s.
Following completion of these major archaeological investigations, excavations and evaluations have been undertaken: around Aggersborg in 1990, 2008-09 (south and west of Aggersborg), and 2011-12 (geophysical at Aggersborg); in 2007-10 in connection with the project 'The King's Fortresses' on Aggersborg, Fyrkat and Trelleborg; in 2019 geo-radar at four fortresses, with Trelleborg following in 2020. Finally, in the late summer 2020 a T-Mac Georadar covered large areas around the Aggersborg Ring Fortress.