275 AD Roman Period
It was the secure location of the harbour that no doubt encouraged the Romans to build their fortress at the top end of harbour some time around 275 A.D and known as Portus Adurni. But in all probability the harbour was already well used for trade with the continent well before the erection of the fortresses, for why else would it have been built if not to protect trade and community. Only when the Saxon pirates became more adventurous and coastal settlements at risk did the walls begin to rise, to take its place in a line of Roman coastal forts that was to become known as “The Forts of the Saxon Shore” It would be prudent not to be content with this image of history, for one can hardly believe that the Roman Army who had a vast experience of warfare would be satisfied with this, for surely at the harbour entrance they would build a watch tower with a beacon close by to raise alarm of intruders entering Spithead or the harbour Channel, for only at the Point (old Portsmouth) can the harbour and the approaches to Spithead can be clearly seen. It may well be that this was the origins of that early settlement at old Portsmouth, it is most certain the Romans would have seen its strategic value. With the demise of Roman Britain around 410 A.D. the fort fell into disrepair.
Construction of Portchester Castle by Carausius, Emperor of Britain, as one of the “Forts of the Saxon Shore” which stretched from Norfolk to Portchester. Carausius was Emperor of Britain from 287 to 293 when he was assassinated. Portchester was the fleet based for the Roman galleys of the “Classis Britannica” … the fleet that guarded Britain.