a. Borough origins on Roman sites
The key question here is was there continuous use of borough sites from the Roman occupation of Europe through to the medieval period.
Although many medieval cities developed upon site that had been previously been used by the Romans, the idea that the occupation of a town site is not a feasible one. Important functions differentiate the large Roman cities from the medieval ones.
Roman cities were political or military centres, more consumers than producers of goods and services. A contrast to the medieval city which was a central market place for the area that surrounded it and a place in which manufactured good were produced.
"For continuity to be proved, town life must be shown to have survived the 6th century, and only at York, Canterbury and London can this be suggested." D.H. Hill (see bibliography).
Because of archaeological evidence the question of continuity is limited to a number of sites, at least 30 sites can claim to be towns by the 10th century by virtue of their mint, market or fortification. There are problems with this that some sites, for example Cirencester, defences were only sparsely used. In some cases such as the six sites of Portchester, Horncastle, Lympne, Clausentum, Manchester and Pevensey the Roman use was very small, generally only a fort which can hardly be accredited as an important urban centre.
There are nineteen Anglo-Saxon towns on substantial Roman sites. In three cases there is record of setting up the burh (defended centre) - Colchester, Chester and Worcester. Ilchester can be disregarded leaving fifteen sites.
Archaeological work in most of these places fills in most of the gaps. It can be claimed then that Roman sites are becoming Anglo-Saxon towns but this is not to be equated with continuous usage as an urban centre. Then why reoccupy the same sites? Three factors seem to be the most obvious: Defence, communication and the church.
Biddle (1972) proposed a model for peaceful continuation of authority after the Roman collapse in the southern areas of Germany. The defending Germanic people brought into the towns by the Romans assumed the roles of the Roman administrators, thus "forming the nucleus of the Germanic petty kingdoms ready to receive the waves of new immigrants in the 5th and 6th centuries".
In terms of fortification it is clear that Iron age Hill-forts were useful to both the British and their Saxon opponents. The same must have gone for the walled Roman towns such as at Porchester and Kent.
In terms of communication, the location of Roman sites was very hard for any other sites to be chosen as towns and trade re-awakened. London, Chester, Gloucester and many other sites stand at the foci of Roman roads and early trackways "it can be demonstrated that of the 34 'towns' founded by the west Saxons that went on to be the doomsday Burhs 27 were associated with roman roads in other words communication was a decisive factor in choosing a town site.
Finally there was the effect of the church, it was not seen fit by the Roman mission for bishops to reside in villages they should be at the heart of things. To carry out the evangelization of Britain to great "metropolitans" were set up at London and at York and had 12 dioceses dependant on each. In the Celtic mission areas the sees were in the countryside, as they were in later Anglo-Saxon England. Thus we have sees centred on Canterbury, Rochester, London, Dorchester-upon-Thames, Leicester, Worcester, Sidnacester and York.
When kings set up their forts and towns they chose defensible sites and the walls and ditches of the Roman sites provided these. They chose sites with good communication and if possible with populations. Roman sites were by definition on the Roman road system and often had ecclesiastical centres within them.
In conclusion the answer to whether the site were continuous depends on how the situation is viewed. It cannot be said that the towns were always there but it can be said the same site were reused time and time again.
Barley, M.W.; European towns - Their archaeology and Early history; CC175.E94 Nicholas, D.; The Growth of the medieval city; HT115.N53 Tait; Medieval English Borough; JS3025.T3 Cambridge Economic History of Europe; HC240.C3 v.3
Hal Eccles. 12th October 1998.