Excavations are thought to have confirmed the original perimeter of Anglo-Saxon Oxford.
Oxford Archaeology found the remains of a ditch near Oriel College, indicating where the town’s defences had originally stood.
Experts said the discovery gave credibility to theories that early Oxford was much smaller than the medieval town it later became.
They said it also showed the Anglo-Saxons had built a square perimeter for Oxford based on the model of walled Roman towns like Winchester.
Archaeologists found the 20m-wide ditch ‘deep below the existing ground level’.
Fragments of charred plants in the ditch show it was constructed between 880 and 950 AD, about the time Oxford is thought to have been founded.
Oxford City Council archaeologist David Radford said: ‘This really is a significant breakthrough that helps us get to the next level in terms of understanding Oxford’s emergence and evolution on the boundary between the kingdoms of Wessex and Mercia in the context of the Viking threat from the north in the 9th and 10th centuries.’
