After the Roman withdrawal from Britain in 410, urban life seems to have continued at Venta Belgarum until around 450, and a small administrative centre might have continued after that on the site of the later Anglo-Saxon palace. Ford identifies the community as the Cair Guinntguic (“Fort Venta”) is listed by Nennius among the 28 cities of Britain in his History of the Britons. Amid the Saxon invasions of Britain, cemeteries dating to the 6th and 7th centuries suggest a revival of settlement. The city became known as Wintan-ceastre (“Fort Venta”) in Old English. In 648, King Cenwalh of Wessex erected the Church of SS Peter and Paul, later known as the Old Minster. This became a cathedral in the 660s when the West Saxon bishopric was transferred from Dorchester-on-Thames. The present form of the city dates from reconstruction in the late 9th century, when King Alfred the Great obliterated the Roman street plan in favour of a new grid in order to provide better defence against the Vikings. The city’s first mint appears to date from this period. In the early 10th century there were two new ecclesiastical establishments: the convent of Nunnaminster, founded by Alfred’s widow Ealhswith, and the New Minster. Bishop Athelwold of Winchester was a leading figure in the monastic reform movement of the later 10th century. He expelled the secular canons of both minsters and replaced them with monks. He created the drainage system, the “Lockburn”, which served as the town drain until 1875, and still survives. Also in the late 10th century, the Old Minster was enlarged as a centre of the cult of the 9th century Bishop of Winchester, Saint Swithun. The three minsters were the home of what architectural historian John Crook describes as “the supreme artistic achievements” of the Winchester School.