Neither Roman nor Saxon settlement is evidenced on the site of the town of Devizes, though Saxons had settled in the surrounds. Its origins are tied to the construction – on a manor of the Bishops of Salisbury where that manor bordered others, held by the king and the bishop – of a castle whose keep and inner bailey topped a steep-sided natural projection from a raised shelf, a good defensible position. The town grew up on that shelf, to the north and, particularly, east of the castle's outer bailey, around which the town's layout curved. The castle was approached by a road from the east, which passed through the town; a later street-name, The Brittox, recalled the castle's bretask or barbican. This initial urban core came to be known, by 1305, as the Old Port and perhaps later as St. Mary's Port (echoing the dedication of the parish church); we might infer from those names the presence of a market. The name of the town itself is suspected as deriving from a Latin term referencing its, or the castle's, plantation on the boundary (divisive line) of the two manors of Bishop's Cannings and Potterne, both held by the Bishop of Salisbury.
The castle is thought to have been built ca. 1080 by Bishop Osmund of Salisbury, kin and chancellor to the Conqueror and one of the architects of Domesday Book, perhaps to control the route through the Vale of Pewsey. Its earliest mention dates to 1106, in connection with the civil war, during which the wooden castle was burned down; the worldly and powerful Bishop Roger le Poer rebuilt it in stone during the 1120s and '30s, on an impressive scale. Made nervous by the bishop's seeming ambitions, King Stephen had Roger arrested in 1139 and his estates declared forfeit; Roger died at the end of the year. The castle changed hands several times during subsequent hostilities. Most of the episcopal estates were restored by Matilda a decade or so later, but she and Henry II retained the castle-town pending a dispute between king and bishop over right of ownership – suggesting the town must have been in existence before Roger's death; the matter was settled by the king acquiring the castle and borough by exchange in 1157.
Devizes remained in royal hands for the rest of the medieval period, the castle serving not only as fortress, but also occasional residence, treasury, prison, and administrative base both for the region and for the town. In the late twelfth and thirteenth centuries, and less frequently in the fourteenth, periodic maintenance and development of the castle must have benefited the local economy; in 1215-16, for instance, 22 carpenters found employment on castle works, some of them likely townsmen. By mid-fifteenth century, however, it was starting to fall into ruin. Yet while the castle put business the way of the townspeople, there was little space within the narrow town for monastic foundations; the two local churches, a modest chapel of the Knights Hospitaller, and a single hospital (first heard of 1314 and never seen to be well-endowed) were the only immediate ecclesiastical clients.
The town that grew up beside this castle was encompassed within an inner ditch (around the outer bailey) and outer ditch topped by palisade, tied in to the castle defences. The narrow settlement included a parish church and marketplace. Alongside this a 'suburb' grew up around a street known (by 1309) as New Port, communicating with the deer park west of the castle. It had its own marketplace by 1378, became a parish in its own right, served by the former castle garrison's chapel, and in the post-medieval period the decommissioned castle's precinct and the park would become part of the parish. Following the emergence of New Port and some expansion of the town (constrained by its defensive earthworks) at the north and south ends, the crescent-shaped town was accessed not only by the road from the east, but perhaps also by assumed gateways at the northern and southern tips.
While an unplanned settlement – possibly a nascent castle-town – formed alongside the original castle and suffered with it the vicissitudes of war, it is only following the rebuilding of the castle that one with borough status is evidenced – and probably at that time provided with its own defences. In 1141 the empress Matilda granted to her burgesses of Devizes, as reward for them having retaken the town from Stephen's men, exemption from tolls throughout the realm and protection for their trading activities. Significantly, the charter was witnessed by the Bishop of Ely, a nephew of Bishop Roger; the nephew had attempted to man the castle against the king after Roger's arrest. Henry II and later kings confirmed this grant; an agreement of 1152 between Henry, while heir apparent, and Roger's successor as bishop, with regard to custody of the castle, suggests the that the borough existed when Roger was still lord of the castle. In 1157 the castle, borough, and lands annexed thereto, were formally ceded to the Crown; they acquired the status of a manorial liberty, exempt from shrieval interference, under the lordship of a farmer of the estate, who was also, in effect, the constable of the castle – increasingly, as time passed, absentees governing by deputy. In 1218 the borough was granted the same privileges Marlborough had received in 1204, which included further exemptions from tolls as well as from feudal obligations, and recognition of a merchant gild; a measure of self-government by bailiffs followed, while a mayor is evidenced by 1302.
The market at Devizes receives its earliest mention in 1228, in the context of the constable of the castle being ordered by the king to cease taking tolls there from men of Salisbury (who were exempt by charter). No market licence is recorded; it must have seemed an unnecessary step at the foundation of the borough, and also once Devizes had become part of the royal demesne. One might have been useful in the context of competition with other markets, had the king not been naturally inclined to favour his own: in 1255 he ordered that a Wednesday market at Lavington, licensed the previous year, be suppressed on the grounds it was damaging to his own ancient market, on Thursdays, at Devizes. However, things were not so clear-cut and the contest was in the king's court in 1260 and still unresolved as late as the Wiltshire hundredal enquiries of 1274, when the local jurors took the opportunity to complain again of the Lavington market being held the previous day to that of Devizes, and made the dubious claim that some king had prohibited any other market within seven leagues of Devizes; the jurors further stated that income from market tolls had, as a result, been reduced from £16 annually to £12, yet a different source records £18 in tolls collected the following year. Meanwhile the owner of the Lavington market had, in 1268, sued a newly-licensed (1266) market at Steeple Ashton for harmful competition. Fairs were a different matter, and the necessary licence was issued in 1223, on an interim basis (during the king's minority), for a June fair at Devizes, although nothing further is heard of it. The lepers of Devizes had received a fair grant in 1208, but this was to be held at their house in Southbroom, neighbouring the borough, and was only a brief event, though profitable enough for bishop and constable to squabble over jurisdiction.
The marketplace of Old Port was an open area between the town's church and the barbican entrance – a space later known as Old Market Place – but overflowed into Market Street and other adjacent streets and later into New Port, where the shambles was located by 1444, at the junction of the later-named High Street and Wine Street. A dedicated area for the fish market is heard of in 1378 and one for horse-trading in 1366. As the castle's military value reduced, its outer bailey offered an expansion area for the town. In the Late Middle Ages what would become the main north-south route (to Bath) through the town was laid out across the former bailey, with regular burgage plots on either side and a wide central stretch to which the main market shifted. This may have been prompted by the original marketplace starting to be lost to encroachment; already by the time of the 1274 enquiry there was complaint about a number of shops installed there, tolerated if not prompted by constable Philip Basset in the 1260s because of the rents they generated – Basset was charged with re-munitioning the castle during the civil war and needed to find funding. Encroachment in the original marketplace continued after the New Port marketplace was established, and even that underwent some infilling in the fifteenth century.
The relatively peaceful fourteenth century saw decline in the maintenance of castle and garrison, which provided employment and business, and there are signs that the borough economy was adversely impacted. A small Jewish community evidenced in the early years of Edward I, which would have helped service economic activities, proved short-lived. Devizes was at no point among the most prosperous or populous of Wiltshire towns. But the growth of local crafts – notably the usual leather-workers and, increasingly, the cloth industry – compensated somewhat. By mid-fifteenth century most of the castle's various roles, mentioned above, had much diminished and the revenues of the liberty were derived more from the rural lands and from the town – the latter being burgage rents, market tolls, court perquisites, and assizes of bread and ale. Even though a decline in toll receipts is evidenced in the latter half of the fifteenth century, Devize's market seems to have done well enough and Leland found it held in high regard in the region.
