Pen Dinas, the largest Iron Age hillfort in Ceredigion, occupies an exceptional position on a coastal hill, which rises towards the western end of a long ridge separating the valleys of the Rheidol and the Paith. Its location is optimal, one of the strongest and most readily defensible hills in the region. This coastal hill boasts precipitous slopes on its seaward (western) side and along the south, but is approached by more gradual slopes across a lower lying saddle of ground to the east and north-east. It sits at the coastal confluence of the rivers Rheidol and Ystwyth, allowing exploitation of a variety of resources within easy reach, including marine resources, seaweed, river fish, good grazing land and soils suitable for arable cultivation, a point well made by Fleure in 1922.
The fort is prominent from many miles around, and was crowned in the nineteenth century by the Wellington Monument (NPRN 32637). The hill of Pen Dinas has two summits, a lower, broader summit to the north, and a higher, more narrow summit to the south. These are linked by a saddle of lower ground known as the isthmus.
Excavations by C Daryll Forde between 1933-7 investigated only limited parts of the fort, but four main phases of development were established. The fort started life as a simple defended site on the north summit, enclosed by a rampart of packed rubble and an outer ditch (Phase I). Some years later after the first was abandoned, perhaps around 400-300 BC, a new fort was built on the higher summit to the south with elaborate gates and a substantial stone-walled rampart with an outer ditch (Phase II). After some time, this fort fell into partial ruin while parts of it were burnt. The fine south gateway collapsed and became forgotten and overgrown. Later, the south fort was re-occupied with new defences built and old ones extensively repaired (Phase III). Finally, additional ramparts were constructed across the isthmus linking both summits, together with a new main gate (Phase IV). At its height (in the last decades before Christ), Pen Dinas was a masterpiece of Iron Age architecture and engineering. The stone-walled isthmus gate stood as high as a two-storey building and was crossed by a wooden bridge supported on four massive timber posts. Manning (1999) provided calculations of the resources consumed by timber-faced hillfort defences, calculating that a 4ha hillfort like Pen Dinas may have required 4680 trees from 76ha of land over its lifetime to provide sufficient timber for vertical posts and tie beams.
Although the south fort was ploughed in historic times, the sites of about a dozen prehistoric round houses can still be seen. These take the form of circular or D-shaped scoops, cut into the bedrock to provide a level platform for building. Several of these 'hut platforms' can be made out in the southern half of the south fort, clustered around the south gate.
The most dateable finds were sherds from an Iron Age jar with stamped decoration around the rim, made about 100BC and similar to 'Malvernian' pottery found on the Welsh borders. A fine glass bead was also found near the north gateway of the south fort, of a pale yellow, translucent colour decorated with three spirals of yellow, opaque glass thread. Other finds included a stone bead, two spindlewhorls, two loom weights, fragments of corroded iron and bronze, and a cache of over one hundred beach or river pebbles probably used as slingshot to defend the fort. Finds from earlier times include a Neolithic stone axe, a Bronze Age palstave and a triangular barbed and tanged arrowhead. A late Roman coin of the emperor Maximian (AD 307) was found in 1930 in a molehill in the south fort. More recently, further slingstones have come to light in various parts of the south fort following the bracken fire of 1999.
