Today Swindon is Wiltshire's largest town, but this is due to modern growth. The historical core of Swindon, known now as Old Swindon and essentially a suburb of New Swindon, stands at the eastern end of a ridge that stretches east-west for about a mile. The top of the ridge has yielded evidence of some occupation since the Mesolithic period, and the small Roman town of Durocornovium had sat on Ermin Street, about a mile to the east, with successive Romano-British and Early Saxon settlements on the hill-top site of Swindon; yet there is no indication it was a candidate for fortification during the period of Viking invasion. Although Swindon is referred to in Domesday, when divided into several manors – the largest (the future town) subsequently known as High or Over Swindon – it is not well documented during the medieval period, and the limited archaeology conducted there has turned up little information from that same period. We can, however, say that there was a church there by mid-twelfth century, and a mill from at least the time of Domesday.

The development of High Swindon into a town can be attributed to the powerful de Valence family who were its lords in the thirteenth century and from whom it came, by marriage, to the Talbots. The energetic William de Valence (d.1296) was one of Henry III's Lusignan half-brothers, for whom Henry undertook to find lands and heiresses to marry. The wife provided to William was a co-heiress of the Earl of Pembroke, whom he had married by 1250, and he had assumed the title of earl himself by 1255, though in official documents he is usually styled as "king's brother". Swindon was one of the many estates that the king gave him as a young man, in the late 1240s, to establish his income, though this favouritism did much to engender the hostility of Henry's barons towards him. William's estates also included: the established but small market town of Bampton (Oxon.); the manors of Shrivenham (Berks.) and Worlington (Suff.), for which William took out licences for market and fair in 1257 and 1258 respectively; Exning (Suff.) a one-time borough that had lost its market ca.1223, but for which William obtained licence for a market and two fairs in 1258; and the borough of Gainsborough (Lincs.), to which William added a fair in 1292. His wife's inheritance included Tenby, a Welsh stronghold converted into a castle-town by an Earl of Pembroke, but vulnerable to Welsh attack, so William restored the town, protected it with a wall in the 1260s, and (acting conjointly with his wife) issued its first borough charter in the 1280s. The charter grants included: exemption from tolls, stallage and feudal services; election of two reeves to hold the borough courts, collect burgage rents, tolls in the town and its port, and a toll imposed on brewing; and the right to hold a three-day fair in August (his son Aymer adding a market grant in 1323).

At the hundredal enquiry of 1274/75 Marlborough's jurors complained that its market had been damaged by one established by William de Valence at Swindon fifteen years earlier, though they did not think it licensed; the jurors of Blackgrove Hundred also reported this 'new market' at High Swindon and equally doubted its validity. In 1281 the charge was addressed though a judicial proceeding of quo warranto, where William's attorney produced a charter of Henry III granting him a Monday market at his manor, and the case was dropped. By 1289 the town was known as Chepying Swindon, to highlight its market role. References to it as a borough with burgages appear in the fourteenth and fifteenth century, and the name of Newport Street, mentioned 1346, is a further indicator of the nature of Swindon.

Its taxation assessment in 1334, and the number of poll tax payers in 1377, suggest Swindon was doing as well, economically, as several other of the smaller towns of the county. Yet the Valence lords never sought to institute a fair in the town, suggesting that trade may have been primarily in staples – one of the seigneurial powers delegated them was administration of the assizes of bread and ale (although it seems later to have been neglected) – and perhaps also livestock, rather than the wool or cloth which more interested merchants from afar. This is perhaps surprising since William de Valence is known to have employed, as early as 1248, Flemish agents to sell cloth manufactured on his estates. At any rate, he and his successors had bigger fish to fry and Chepyng Swindon, perhaps neglected by its lords, never amounted to more than a small market town during the Middle Ages. Like many small Wiltshire towns, there is no sign of further expansion during the Late Middle Ages; its lords seem to have felt that the infrastructure established in the latter part of the thirteenth century was adequate to needs, in terms of population level and commercial viability, and required no further attention, beyond the usual upgrading of the fabric of the parish church.

The new town was laid out around two north-south streets, one (the later High Street) running north to Cricklade, the other south to Devizes. Running west off the High Street were Wood Street and Newport Street; these streets formed three sides of a rectangle encompassing most of the new town; plots laid out along them – particularly Wood and to a lesser extent Newport – have, on old maps, the look of burgage tenements. That a narrow Back Lane ran off the west end of Wood Street is also suggestive of burgage plots. The older village, church and manorial enclosure were on the east side of the High Street, with the Devizes road as the village focus; the High Street may therefore have been a new road created at the same time as the town (although conceivably on an existing track) and gradually superseding the village street (which later disappeared), taking a new, southerly direction towards Marlborough. A large marketplace, reflective of the founder's aspirations, opened off the east side of the High Street, and lies between two cross-lanes connecting the old and new focal streets, so it is ambiguous whether it originated with the town or with the village.