Noviomagus Reginorum was Chichester's Roman heart, very little of which survives above ground. It lay in the land of the friendly Atrebates and is in the early medieval-founded English county of West Sussex. On the English Channel, Chichester Harbour, today eclipsed by Portsmouth Harbour, lies 4.5 miles (7.2 km) to the south.
The name of the town is given as "Noviomagus" in Ptolemy and "Navimago regentium" in the Ravenna cosmography. This is believed to be a Latinization of a Brittonic placename meaning "new plain" or "new fields", in other words a clearing in woodland. Its epithet is drawn from the name of the inhabitants — reconstructed variously as Reginorum, Regnorum, Regnentium, Regnensium or Regentium— in order to distinguish it from other places with the same name, notably Noviomagus in Kent. The Regini were either a sub-tribe of the Atrebates or simply the local people designated the 'people of the Kingdom' by the Roman administration. In the 2nd-century Antonine Itinerary register of Roman roads, the name is abbreviated to "Regno".
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. Their timber barrack blocks, supply stores, and military equipment have been excavated. The camp was in the territory of the friendly Atrebates tribe and was only used for a few years before the army withdrew and the site was developed as a Romano-British civilian settlement.
Kilns have been found from the building in the early 1950s, and a bronze works from the Neronian or early Flavian period; and a dedication to Nero is dated to A.D. 58. The River Lavant was diverted to provide a public water supply. The town served as the capital of the Civitas Reginorum, a client kingdom ruled by T. Claudius Cogidubnus. Cogidubnus almost certainly lived at the Palace of Fishbourne, a mile to the west. He is mentioned on the dedication stone of a temple to Neptune and Minerva. Other public buildings were also present: public baths are beneath West Street, an amphitheatre under the cattle market (this suffered stone-robbing in the late second century, by which time it was presumably no longer in use), and a basilica is thought to have been on the site of the cathedral.
The town became an important residential, market and industrial centre, producing both fine tableware and enamelwork. In the 2nd century the town was surrounded by a bank and timber palisade which was later rebuilt in stone. Bastions were added in the early 4th century and the town was generally improved with much rebuilding, road surfacing and a new sewerage system. There were cemeteries outside the east, north and south gates.
By the 380s, Noviomagus appears to have been largely abandoned, perhaps because of Saxon raids along the south coast. According to the Anglo-Saxon Chronicle the town was eventually captured towards the close of the 5th century, by the legendary Ælle of the South Saxons. It was renamed after his son, Cissa, and probably retained as a royal residence.