Castell Grogwynion is a broadly rectangular fort in plan, measuring nearly 170m east-west by approx. 100m north-south. The topography is pronounced on site, with the terrain falling nearly 29 metres from the summit of the western outcrop to the approaches of north-east gateway. The fort has one main gate at the north-east point, defined by an impressive free-standing walled bastion associated with a complex annex arrangement to the entry. However, there are at least two further gateways on the northern defensive circuit suggesting a long and complex history to the development of the fort, perhaps beginning with a smaller hillfort enclosing the highest point - the western outcrop - with subsequent enlargement onto the hillslopes below and to the east.

The hillfort is depicted at a small scale on the 1834 Ordnance Survey map of the region, and was competently surveyed by Ieuan Hughes for his 1926 survey of the regional hillforts (in the Transactions of the Cardiganshire Antiquarian Society). It was surveyed for the Ordnance Survey in 1974 at 1:2500 scale but this omitted much useful topographic and earthwork evidence. Toby Driver for the Royal Commission surveyed the hillfort in 2002 with Richard Knisely-Marpole towards his PhD study, but the fort was comprehensively re-surveyed by Louise Barker of the Royal Commission in 2012 at which point new evidence emerged about the possible phasing of the fort and important post-medieval activity on site. The hillfort was sampled in 2011 with a pXRF scanner by Keith Haylock and Toby Driver to assess the presence of metal working evidence from the fort, and this was followed in 2012 with a new geophysical survey of the house platforms and a high lead anomaly on the northern terrace by ArchaeoPhysica LTD, funded by the Royal Commission.

The overall fort is arranged in a series of `steps? or compartments, with the main gate positioned at the lowest (north-east) point and the visitor progressing upwards (westwards) through the fort, ultimately towards the high western outcrop at 289 metres O.D. To gain entry through the main, possibly latest, gateway at the north-east point one must pass first a prominent bastion or slinging platform on the left (north) side, then turn through 180 degrees passing into the lowest 'compartment' of the fort interior via an annex. Moving uphill one comes to a central part of the fort formed between a high, modified slope on the west side, by outcrops on the east, and by the inner terrace of the main facade on the north, which turns in through 90 degrees to the rear of the outcrops. A climb uphill brings you to the last internal area below the outcrop, a steep natural terrace partly modified on the north and east sides. On this last terrace are the remains of three denuded house platforms, confirmed by topographical and geophysical survey in 2012.

The high western outcrop, encircled with well-built low ramparts set on good foundations, can be interpreted as the earliest phase of enclosure on site, once a true inland promontory fort. A prominent knoll over a precipitous drop at the south-west point of the fort, overlooking the Ystwyth Valley, is also plausible as a 'lookout' point.

Medieval or post medieval evidence emerged during the 2012 survey by Louise Barker of a low earthwork platform, centrally located in a sheltered position on the northern terrace and surrounded to the south and west by plough cultivation. There are also traces of exploratory mining cuts along the northern part of the fort. The latest intervention on site appears to be a small box-trench within the north-western ramparts, flanked by a low spoil tip, which seems to be an unrecorded antiquarian excavation. See longer discussion about the north-east gateway in Driver, T. 2013, pages 74-76, and discussion about the hillfort on pages 101-102 & 123-125.

The 2013 excavation: Between 6th to the 10th October 2013 the Early Mines Research Group (EMRG), working with the Royal Commission (RCAHMW) and a doctoral student from Aberystwyth University (Keith Haylock), undertook a trial excavation on the northern terrace of Castell Grogwynion hillfort. This excavation was to investigate a high lead/burning anomaly and low earthwork first identified during surface sampling, which was subsequently confirmed as a probable furnace site following a Royal Commission-funded geophysical survey carried out in 2012.

The lead soil anomaly was identified by Keith Haylock, a KESS (Knowledge Economy Skills Scholarship) PhD Student within the Department of Geography and Earth Sciences (DGES) at Aberystwyth. Grid sampling of topsoil was carried out in 2011 across a number of transects within the hillfort interior using a Portable X-Ray Fluorescence Analyser (pXRF) to identify any evidence of metallurgy contemporary with the occupation of the hillfort . The EMRG was asked by the Royal Commission to assist with the investigation of this apparently archaeometallurgical/ mining-related feature.

Contrary to expectations the excavation found that the lead anomaly was diffuse and located just within the upper 25cms of the soil profile, where it was associated with charcoal and 575 sherds of post medieval pottery (late 17th- mid 18th centuries), a highly unusual and interesting find for the county. Other than that there appeared to be no evidence for burning, nor of a furnace structure; and no indication either of a source for the locally high lead contamination (of 6 to 40 x the background level of 50 -70 ppm Pb (lead)). See Timberlake et al. 2013.

T. Driver, RCAHMW, 2014 and 2018

Main sources:

Davies, J. L. and Hogg, A. H. A., 1994. The Iron Age. In: Davies and Kirby, eds., 219-233.

Davies, J. L. and Kirby, D. P. eds., 1994. Cardiganshire County History. Volume 1, From the earliest times to the coming of the Normans. Cardiff: University of Wales Press.

Driver, T. 2013. Architecture, Regional Identity and Power in the Iron Age landscapes of Mid Wales: The hillforts of north Ceredigion. BAR 583, Archaeopress.

Driver, T. 2016. The Hillforts of Cardigan Bay. Logaston Press.

Haylock, K. 2015. The Relationship between Iron Age Hill Forts, Roman Settlements and Metallurgy on the Atlantic Fringe. Unpublished PhD thesis. Aberystwyth University.

Hughes, I.T. 1926. A Regional Survey of North Cardiganshire Prehistoric Earthworks, Transactions of the Cardiganshire Antiquarian Society, IV, 22-56.

Timberlake, S., Haylock, K, Driver, T, Barker, L, Andrews, P, Craddock, B, Gilmour, A and Mepham, L. 2014. THE STRANGE CASE OF A MYSTERIOUS LEAD ANOMALY: CASTELL GROGWYNION HILLFORT, CEREDIGION. Archaeology in Wales 53 (2013), 27-36