The name element -magus generally meant a large flat area, such as a plain or a marketplace, but it could get reinterpreted and respelled as Latin magnus 'great'.
The settlement was first established as a winter fort for the Second Augustan Legion under Vespasian (the future emperor) shortly after the Roman invasion in AD 43.
The camp was in the territory of the Atrebates tribe, whose rulers were friendly to the Romans, and was only used for a few years before the army withdrew and the site was developed as a Romano-British civilian settlement.
Bastions were added in the early fourth century and the town was generally improved with much rebuilding, road surfacing and a new sewerage system.
However, although by the 680s the area between Chichester and Selsey had become the political and ecclesiastical centre of the Saxon kingdom with the kings residence in Orreo Regis (Kingsham), south west of Chichester, and Wilfrid's religious centre in Selsey, the archaeology does not support Anglo-Saxon settlement of the central city area until the ninth century.