This article revisits the archaeology of the Viking-age settlement and ring fortress at Aggersborg, Denmark, based on a large-scale geophysical survey using magnetic gradiometry and ground-penetrating radar, as well as legacy excavation data. Late 10th-century Aggersborg, the largest known fortress in Viking-age Scandinavia, commanded a key position at the narrow strait of the Limfjord, a principal sailing route between the Baltic and the North Sea. Previous excavations established that this location was on the site of an earlier settlement, which was burned-down prior to the construction of the fortress. The character and extent of this prior activity, however, have hitherto remained ill-defined. The geophysical survey identifies previously unknown elements of the fortress structures and elucidates the extent and character of the earlier settlement. The analysis is combined with a comprehensive reconsideration of primary data from early excavations, and demonstrates how this evidence can guide the interpretation of geophysical data to yield a detailed reassessment of spatial structure, and even suggest chronological phasing. The excavation trenches show dense traces of occupation with a large number of sunken-featured buildings (SFBs). Anomalies consistent with similar features are mapped in the geophysical surveys, and their distribution is shown to complement results from the excavations, demonstrating the important contribution of non-invasive survey to our knowledge of scheduled monuments. The surveys suggest that the total number of SFBs may be as high as 350, equal to or exceeding the largest number of such buildings previously identified at any site in Scandinavia. The ring fortress, by implication, must have replaced a site of particular function or importance, albeit of a very different organisation. An interpretation of the communication landscape is combined with a visibility analysis to argue that the long-term significance of the site relates to the potential of the location as a central place. These new observations transform understanding of a key site in Viking-age archaeology, and of the choice of location and the purpose behind the exceptional fortress. They offer a case for a reassessment of the much-discussed group of so-called ‘Trelleborg’ fortresses, thus adding new substance to models for understanding the political and tactical role of fortified places in Early Medieval Europe.