The medieval remains of the castle including the motte, St George's Tower and crypt, are Grade I listed buildings and a Scheduled Monument.
According to the Historia Ecclesie Abbendonensis (Abingdon Chronicle),[1] Oxford Castle was built by the Norman baron Robert D'Oyly the elder from 1071 to 1073.
[2] Oxford had been stormed in the invasion with considerable damage, and William directed D'Oyly to build a castle to dominate the town.
[14] The new curtain wall incorporated St George's Tower, which is built of coral rag stone, 30 by 30 feet (9 m × 9 m) at the base and tapering significantly toward the top for stability.
[24] In 1074 D'Oyly and his close friend, Roger d'Ivry had endowed a chapel with a college of priests, which is presumed to be the structure in question; at an early stage it acquired a dedication to Saint George.
The ten-sided stone shell keep, 58 feet (18 m), constructed in the 13th century to replace an earlier wooden structure, closely resembled those of Tonbridge and Arundel Castles.
[28] Stephen responded by marching from Bristol in the Autumn of 1142, attacking and seizing the town of Oxford and besieging Matilda in the castle.
[25] Stephen would have had difficulty in supplying his men through the winter period, and this decision shows the apparent strength of Oxford Castle at the time.
[29] Finally in December, Matilda responded by escaping from the castle; the popular version of this has the Empress waiting until the Castle Mill Stream was frozen over and then dressed in white as camouflage in the snow, being lowered down the walls with three or four knights, before escaping through Stephen's lines in the night as the king's sentries tried to raise the alarm.
[25] Robert had died in the final weeks of the siege and the castle was granted to William de Chesney for the remainder of the war.
[40] Ralph Agas's map of Oxford in 1578 shows that by then, while the curtain wall, keep and towers remained, the barbican had been demolished to make way for houses.
Hassall, 1976, states that by 1600 the moat was almost entirely silted up and houses had been built all around the edge of the bailey wall,[41] although this is contradicted by the castle's appearance in John Speed's map of Oxford, 1605.
[46] In the event, Oxford saw no fresh fighting; early in the 18th century, however, the keep was demolished and the top of the motte landscaped into its current form.
[50] A view of the castle published in 1769 in the work "England Displayed" by P. Russell and Owen Price is of interest in that it shows the appearance of the chapel attached to St George's tower prior to its demolition in 1794, as well as the motte and some then-surviving portions of the curtain wall including an arch or gateway in the wall immediately to the north of the tower.
[60] Harris gained a reasonable salary as the new governor and used convict labour from the prison to conduct early archaeological excavations at the castle with the help of the antiquarian Edward King.
[61] In the 19th century the site continued to be developed, with various new buildings built including the new County Hall in 1840–41 and the Oxfordshire Militia Armoury in 1854.
[33] The inmates included children, the youngest being a seven-year-old girl sentenced to seven days hard labour in 1870 for stealing a pram.
[64] The full extent of the original castle is somewhat obliterated today, especially with the intrusion of the newer County Hall into the eastern side, while New Road runs over the location of north-east portion of the curtain wall with its two square towers; nevertheless the position of its outer perimeter moat is approximated by portions of New Road, Castle Street and Paradise Street (refer map at right), while the remains of the original Barbican lie underneath the modern Westgate shopping centre.
[66] As at 2018, guided tours of the surviving medieval and 18th-century portions are available to visitors via a commercial operator, Heritage Projects (Oxford Castle) Ltd, with opening hours and pricing available via their website.