Pen-y-Dinas is a later Prehistoric type hillfort set above a narrow stream valley leading up to the uplands from the coastal plain of Dyffryn Ardudwy. The fort occupies the sloping tip of a west-facing spur on the north side of the valley. It is a roughly oval enclosure whose interior slopes down to the west. It is about 60m east-west by 50m, and is defined by a tumbled stone wall or rampart, 4.0-5.0m wide, now mostly turf covered. The wall is best preserved either side of the slightly inturned west-facing entrance. The walls were ditched about and on the southern side, overlooking the valley, there are indications of a second outer rampart which encloses a small annex at the west end. A trackway, sometimes a holloway and sometimes flanked by tumbled stone walls, leads from the inner entrance, through the annex and runs for some 100m to the north-west.

Excavation trenches were dug in 1919 when traces of a stone-walled building some 8.0m across were recorded, terraced into the slopes in the interior. An iron 'rivet' was recovered from a deposit behind the rampart wall, as was material that appears to have been idenitifed as VCP - Very Coarse Pottery - used in the Iron Age for salt containers probably produced in Cheshire.
There are extensive remains of medieval and post medieval settlement below the hillfort to the north and west (see NPRN 89095-6).

Source: Crawford in Archaeologia Cambrensis sixth series 20 (1920), 116-125
Source: Bowen & Gresham 'History of Merioneth I' (1967), 150-1
Source: Savory 'Guide Catalogue to the Early Iron Age Collections' (1976), National Museum of Wales, 69 No. 85

John Wiles, RCAHMW, 31 January 2008