Mapping Isca: the legionary fortress at Caerleon and its environs

This on-going research project is intended to provide a better understanding of the layout of the Second Augustan Legion’s fortress of Isca, its garrison and the population of the settlement beyond the fortress walls (the canabae).

Roman Caerleon has been the focus of work for more than 150 years and archaeologists from the National Museum of Wales, Cardiff University (in its various incarnations) and, more recently, the Glamorgan-Gwent Archaeological Trust have all excavated in and around the fortress. The earliest investigations took place in 1843, though it was the intensive series of campaigns of the 1920s to 1960s that established the internal layout of large parts of the fortress as well as its chronology. Further excavations in the 1970s and 1980s added considerably to our knowledge of Isca and today the base of the Second Augustan Legion at Caerleon is one of the most well known legionary fortresses in the Roman Empire. Yet there are large areas of the fortress that have not been the subject of any archaeological work and about which a great deal remains to be discovered. In particular, almost nothing was known about the south-western corner of the fortress between the via praetoria and the dextral side of the via principalis. Obtaining a more detailed and reliable plan of Isca and the civilian canabae around it is an essential part of understanding Roman Caerleon and this objective was highlighted as a priority in the Caerleon Research Framework.

In 2006 staff and students from Cardiff University carried out a magnetometer survey of Priory Field in the south-western corner of the fortress. The results were spectacular and showed the potential of remote sensing techniques to locate and map the buried remains of Roman Caerleon. Magnetometer and resistivity surveys of other parts of the fortress and the extramural settlement on its western side have been undertaken annually since 2006 and some of the results are described below.

Priory Field surveys

In the spring of 2006 Cardiff University, as part of its undergraduate 'Survey and Prospecting' course, commissioned GeoArch to lead a geophysical survey of Priory Field occupying the entire south-western corner of the Roman fortress. Only limited archaeological work had been undertaken in Priory Field in the past and, consequently, this part of the fortress is shown as a blank area on most plans of Isca (though it was assumed that barracks must have occupied some part of it).

The 2006 magnetometer survey located eight barrack blocks arranged along the fortress' south wall, as well as three very large rectangular buildings (their plans are characteristic of Roman military granaries), along the west wall towards the porta principalis dextra (west gate). An extensive area of open ground to the east of these three buildings was flanked on its opposing side (i.e. the north-eastern side of Priory Field) by one wing of a much larger square courtyard building that was subdivided into a series of rooms of approximately the same size. The magnetometer picked up strong readings towards the southern corner of this building that may or may not be evidence for burning.

The interim report of the 2006 geophysical survey is available in the downloads section

For the full publication of the Priory Field survey results and their interpretation, see:

Guest, P and Young, T. 2006 'Mapping Isca: geophysical investigation of Priory Field, Caerleon', Archaeologia Cambrensis 155, 117-33.

A resistivity survey undertaken in 2007 of the northern part of Priory Field revealed the three rectangular granaries and the courtyard building in even more detail. The survey extended into the car park of the Priory Hotel where the far side of the courtyard building was located, confirming that it was almost exactly square. The plan of the building, although incomplete, suggests it was possibly a store building or horreum.

The geophysical surveys were followed by the excavation of a series of evaluation trenches in 2007, directed by Andrew Gardner and Peter Guest with students from Cardiff University and UCL. Six trenches were opened in Priory Field to investigate a possible rampart building, rooms in two barrack blocks, one of the military granaries, and the open 'yard' area between the granaries and the large courtyard building. The results of these evaluations can be found in the interim report in the downloads section.

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School Field / Gollege's Field surveys and evaluation

A second geophysical survey was undertaken within Isca in the spring of 2007, this time of School Field and Golledge's Field between the via principalis and the barrack blocks in Prysg Field to the northwest. Like Priory Field, little was known of School Field and previous plans of Roman Caerleon left the area blank (though it was suggested after the 2006 survey that the Caerleon Endowed Schools' playing field might have contained more military granaries to complement those discovered in Priory Field).

The magnetometer survey of School Field located a very large courtyard complex, as well as part of the barrack blocks for the legion's First Cohort (these extended northwards from the via principalis in Golledge's Field). The main complex occupied an area approximately 70 metres square and appears to have included an aisled basilical hall with large tower-like rooms at each end, and other long buildings subdivided into large rooms around the remaining three sides of the central courtyard. The entranceway, half way along the southern range, provided access into the complex, which was probably the fortress' main fabrica where metalworking would have taken place on an industrial scale. The magnetic readings recorded for this complex are much higher than for the barrack blocks immediately to the south, indicating extensive in-situ burning or the spreading out of metalworking debris. The rectangular structure in the northern part of the courtyard could have been for the storage of water (of which large quantities are required in the smelting and smithing of metals), while the small rooms or cells along the external wall side of the eastern range may have been for storage.

Excavation of Golledge's Field by Nash-Williams in the 1930s revealed a series of buildings along the northern side of the via principalis towards the porta principalis dextra (west gate). This work was never fully written up and only a summary plan of the buildings was published, though it was thought that the buildings correspond to the centurions' quarters for the First Cohort's barrack blocks. The magnetometer survey conducted in Golledge's Field in 2007 found that the area was heavily disturbed and, consequently, it was not possible to locate by geophysical prospection the buildings planned in the 1930s.

A trench was excavated across the full width of the central building in Golledge's Field in the summer of 2007 (part of the trial excavations directed by Andrew Gardner and Peter Guest). The purpose of this trench was to find out how Nash-Williams had excavated the site, if the buildings’ plans had been copied correctly on to the plan of the fortress, and whether or not the surviving archaeological deposits might warrant further excavation in the future.

The results of this excavation and the evaluations in Priory Field are available in the downloads section


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Preliminary results of magnetometer survey of Priory Field, 2006. (© GeoArch).

Results of the resistivity survey of Priory Field 2007 (© GeoArch)

Results of magnetometer survey of School Field, 2007. (© GeoArch).

Interpretation of Priory Field and School Field magnetometer surveys. (© GeoArch).

Download hi-res version (1.2MB)

Excavation trench in Golledge’s Field 2007 (© Cardiff University / UCL)