Thousands of years ago glacial action carved a mile-wide gap in this ridge. Here, the River Witham met the westward-flowing River Till and a lake eventually formed, now known as the Brayford Pool. The combination of routeway and river formed a natural site for settlement, and our earliest evidence of settlement belongs to the first century B.C., with some finds dating from even earlier The Roman Arrival Around the middle of the first century AD the Ninth Legion of the Roman army took over a large area, seeing control of the natural gap as essential on the route of conquest northwards. The river could be defended here, and be useful for the army's own supply purposes. First a fortress was built on the hilltop.
Excavations indicate an earthen bank held in place by timbers and fronted by a v-shaped ditch.
Twenty or thirty years later, the Ninth went on north to build York .