Modern day Dorchester-on-Thames is only a village of some 3,000 inhabitants in the county of Oxfordshire, situated on the banks of the river Thame near its confluence with the Thames.
It was previously the Romano-British walled town of Dorcic and lay on the Roman road that led from Silchester to Alchester, a significant market centre with a Roman military camp located nearby.
Nearby there is an early iron age hill fort of about ten hectares in extent, and also the much larger, late iron age valley fort of Dyke Hills. Within the Dyke Hills fort there is also an early, that is early fifth century Germanic mercenary graveyard. This has led to speculation that this part of the Thames Valley was garrisoned by a troop of Germanic foederati, who intermarried with the local Romano-British community and stayed behind after the departure of the Romans to act as their protectors.
Certainly in the later fifth century Dorcic, re-christened 'Dorcic-ceaster' (and hence later Dorchester), became the capital of the Anglo-Saxon kingdom of the Gewissae and it was there in the year 635 that Birinus, sent to England by Pope Honorius I to further the cause of Christianity in Britain, converted and formally baptised Cynegils, king of the Gewissae in the presence of his godfather Oswald of Northumbria.
A cathedral church was built to serve this new diocese of the English church
and Birinus himself was buried there in 650. But encroachments by Mercia forced the Gewissae to re-locate their capital, both spiritual and temporal, to Winchester (and in the process re-invent themselves as West-Saxons)
Under Mercian rule from around the year 670, Dorchester lost most of its secular significance; in religious terms it now came under the jurisdiction of Lindsey and Leicester. Until around the 870's that is, when the pressure of Danish Viking incursions resulted in the transfer of the bishopric back to Dorchester. There it remained for a couple of centuries, when the diocese stretched from the Thames to the Humber, but by 1086 the bishopric had been moved once again to Lincoln and Dorchester slumped back into relative insignificance.
The old cathedral was refounded in 1140 as a monastery for the Canons of the Order of St. Augustine by Alexander, the bishop of Lincoln, and thereafter became known as Dorchester Abbey. The Abbey escaped the worst effects of the Dissolution of the Monasteries due to the intervention of a local magnate Richard Bewfforeste and was retained for use as the local parish church.
Dorchester Abbey remains standing today and is the probably the main tourist attraction of the area and also features the Dorchester Museum and the Abbey Tea rooms.
This Dorchester is now known as 'Dorchester-on-Thames' in order presumably, to avoid confusion within the other Dorchester which is in Dorset.