Gaer Penrhôs castle was built in 1149, some say by Cadwaladr, son of Gruffydd ap Cynan. Built within an earlier Iron Age hillfort, which has been used as a bailey. The earthworks are well preserved and of considerable strength.

Gaer Penrhôs Hillfort – Llanrhystud Castle

Extract from Royal Commission on the Ancient and Historical Monuments of Wales (RCAHMW), “National Monuments Record of Wales”

The remarkable earthworks of Gaer Penrhôs crown the summit of a precipitous hill above the junction of Afon Wyre Fawr and Afon Wyre Fach a mile to the east of the village of Llanrhystyd. It is quite a spectacular site, forming a vantage point looking deep into the monuntains about the head of the Ystwyth valley and westwards and southwards over Cardigan Bay. Evidently during the Iron Age the whole of the hill top was encircled by a high rampart of considerable strength enclosing an irregular kidney-shaped area with a simple entrance on the N.W. Its maximum lenght is approximately 800′ and with a maximum width of 400′. The interior is tolerably level but with a general slope to the W. The rampart is well preserved on the on the N. and E., less so to the S., while on the W. its position has been usurped by a hedgerow boundary of dry stone walling. The rampart has a maximum external height of 25′ and internal height of 4′. It appears to be largely scarped from the hillside. On the S. little but scarp remains. On either side of the entrance are traces of an external fosse, but for the greater part of the enceinte any indication of a fosse is absent. Forming part of and projecting from the S.W. enceinte is a massive ring motte of Norman date. This is not sited at the highest portion of the defences, which is strange, and its situation is such that any view to W. and N. is completely obscured by higher ground within the area of the hill fort. The ring motte has an appearance of being square in plan, with rounded angles. Its surrounding rampart rises 8′ above the interior, and some 15′ to 25′ above the rock-cut fosse which separates it from the interior of the hill fort. On the E. the scarp is over 45′ high. There is an entrance on the S. which communicates not with the interior of the hill fort but outside its defences. There are no certain traces of any attached bailey unless the whole area of the hill fort was so used. A short distance within the entrance of the hill fort are traces of a slight rock-cut ditch which seems to have marked only a former hedgerow boundary. Outside the defences of the hill fort to the south are the remains of two scarpings which perhaps indicate additional defences on this side. On the N. and W. of the entrance are indications of a rectangular enclosed bounded by a slight bank, but whether it is contemporary with the hill fort or ring motte is most uncertain. Below the S.W. corner of the motte there is a small platform defined by a scarping and below that a sinking which seems to have been an old quarry pit.

The ring motte probably represented the castle founded by Richard de la Mare a follower of Richard de Clare in 1110. Seized and captured in Gruffyd ap Rhys devastation of 1116. Again destroyed by Owain Cadwallader in 1136. In 1143 it was taken by Hywel but in 1144 it was again retaken by Cadwallader. There seems to have been two castles of Llanrhystyd and this was one of them. It is stated that Cadwallader built a castle at Llanrhystyd in 1149 and handed it over with a neighbouring portion of Ceredigion to his son Cadfan. Hywel deemed the moment a suitable one for aggression and seized his cousin and possessed himself in 1150, of land and castle, making himself lord of the whole province.