Parc-y-Castell is an embanked and ditched ringwork, with the remains of a rectangular outer bailey. It survives in a concealed position on the north slopes of the River Alun, just west of St Davids, between the city and Porthclais harbour. Its origins are unclear and there is no recorded history, although some historians consider it may have been built by the bishops around 1115 to protect the early cathedral precinct from attack.

The ringwork measures some 40m by 32m internally, resting on natural valley scarps above the Afon Alun to the south-east, with a probable scarp-edge entrance to the north-east, showing a possible building platform within. A rectangular enclosure, 45m by 40m, banked and ditched, adjoins on the north-east. Parc-y-Castell is possibly associated with a series of linear banks (NPRN 400105), the southern-most of which runs uo to its south-western outer ditch.

Sources include:
Driver, T. 2007. Pembrokeshire, Historic Landscapes from the Air, RCAHMW, pg 103, Figure 154

T. Driver, 28 June 2007.