Type: Early Roman Naval Base
WNW (24) to CLAVSENTVM (Bitterne, Hampshire)|
E (4) to NOVIOMAGVS (Chichester, West Sussex)
Ptolemy's Geography published in the early-second century AD, provides us with a fairly accurate picture of The Island of Britain at the time. In book II chapter ii, Ptolemy describes the coastal outline of Britain, giving the latitude and longitude of a number of promontories, river-mouths, peninsulas and bays, and his paragraph headed "the description of the south side below which is the Oceanus Britannicus", gives the names of several features on the southern English coast. Here, between the rivers Iscas and Alaunus to the west and the Trisantonis¹ to the east is a place known as Magnus Portus or "the Great Harbour", which must refer to the natural harbours between Portsmouth and Chichester.
There is no Latin inscription on stone recorded in the R.I.B. for the Bosham Harbour site, and the only piece of evidence we have of its former importance is a legionary helmet which was dredged-up from Bosham harbour many years ago. This helmet has been described as a 'Jockey Cap' type, which was phased-out of service with the Roman army in late-Claudian times, in favour of an improved design with better neck and cheek protection. This helmet now resides in the Lewes Museum.
"Helmets similar to that at Lewes are a sure sign of Claudian military activity." (Dudley & Webster, p.201, #22)
It has been claimed by some that the known Roman harbour at Bosham, which is naturally split into three major tributaries, was the original landing site of the Roman invasion of AD43, but this cannot be reconciled for a number of reasons. For a start, the Romans were very averse to sea-borne operations of any kind, especially on their north-western seaboard which was subject to that very un-Mediterranean problem of tidal currents, and they would have been very wary of extending their initial supply-lines this far along the southern coast. Secondly, dio mentions a two day long battle at a major British river before the Thames, and there is no likely candidate between Chichester and London.
Bosham Harbour and the nearby legionary depot at Chichester may be assigned to the secondary thrust of the invasion period, probably early in 44, when the Second Legion Augusta commanded by the future emperor Vespasian saw active service against the Durotriges and the Belgae tribes in southern England. This conflict is touched upon in Suetonius (Vespasian IV.1).