Swein Forkbeard at St Michaels' tower

Priscilla Waugh strolls down Cornmarket Street to St Michael's tower, a defensive stronghold against Viking assaults during the reign of Ethelred II

Little remains visible of Anglo-Saxon Oxford. Until, that is, the city council embarks on one its periodic attempts to modernise the surface of ancient Cornmarket.

Then, a rabbit warren of cellars and passages frustrates any possibility of building the kind of surface that will meet the exacting requirements of today and, having absorbed an extraordinary amount of money, the whole thing is covered over again with tarmac and chewing gum and consigned to the 'Too Difficult' box for a bit longer.

Watercolour by David Langford

But if you descend at Folly Bridge to the river bank, you will see the remains of an Anglo-Saxon stone bridge supporting the north-south road to Abingdon. Such a bridge, together with the lack of evidence of a major town might suggest that Oxford was simply a convenient Thames crossing for the local wool trade. The city's oldest remaining building, though, speaks of a community of some substance.

The imposing tower of St Michael in Cornmarket dates from the 11th century, a solid structure of four storeys, built of coral rag from the local Cumnor Hills. The stone is a tough one, not greatly given to ornament, but the function of St Michael's tower was, many say, defensive.

Viking assaults on England, as on other parts of the world, had begun towards the end of the 8th century and continued throughout the reign of Ethelred II ('The Unready') between 978 and 1016. The tower, being situated at the north gate of the city wall, its church probably stood south of the tower, so that people could enter in safety and take shelter, if need be, in the upper storeys.

Those less sure of the tower's defensive function point to the door leading to the outside of the wall, maintaining that this was clearly an adjunct to the church, for surely such an entryway would make nonsense of any claim to security. Anyway, they say, were the worst of the Viking invasions not over by now? It seems likely that by the time the tower was built, Cnut, son of Swein Forkbeard, was on the throne.

But how long were Viking memories? In 1002, King Ethelred, having heard a rumour that the Danes planned to murder him and seize his kingdom, ordered that, on St Brice's Day, November 13, all Danish men living in England should be killed.

By this time, the vast majority of Danes in England were living perfectly peaceful and law-abiding lives, many having obligingly embraced Christianity and married English women.

So Oxford's Danes, among whom, it seems possible, Gunnhild, King Swein's sister, with her husband and two sons, took refuge in St Frideswide's Church, which stood on the corner of High Street and Turl Street.

Ethelred's citizens of Oxford, dutiful to the end (and how prophetic a word is that!) then barricaded the doors of St Frideswide's from the outside and burned the sheltering Danes alive.

Whether or not Swein had a personal score to settle with Oxford, one thing is certain: when next he and his army visited, in 1013, having en route "wrought the greatest evil that any raiding army could do" Oxford's citizens immediately submitted.

What had happened in the intervening years? Perhaps, even by now there was little left of their Anglo-Saxon town.

October 2005 Oxfordshire Limited Edition