Pen'r-allt hillfort was discovered during RCAHMW aerial reconnaissance on 14 March 2003, and recorded as an interesting earthwork and probable hillfort. This was confirmed during a subsequent site visit on 5 November 2004. A detailed earthwork survey of the monument was carried out by RCAHMW in November 2006.

The hillfort occupies a prominent, rounded ridge at 291m above sea level, which overlooks the steep valley of the Afon Clywedog from the north and generally commands the confluence of the Clywedog and the Severn just over a kilometre to the south, at Llanidloes. This ridge is aligned south-west to north-east and is traversed by outcropping lines of rock.

The most prominent surviving remains of the hillfort are the ramparts which defend the main gateway located in the north-east corner of the fort. These comprise three parallel banks, constructed in a slight hollow, which measure up to 40m in length and stand a maximum 2.5m high, each associated with a less well defined outer ditch. Two further lines of defence, some 35m in length and created through the modification of the steep valley slope, define the eastern edge of the fort, to the south of the gateway.

The interior of the hillfort is likely to have been confined to the level summit ridge, roughly 70m north-south by 50m east-west. There is no evidence for any surviving defences around the southern and western edges of this ridge, and it is likely that the precipitous valley slopes would have provided natural defence, and thus a less substantial barrier, such as a palisade, may have been constructed. Ploughing across this ridge, as indicated by a number of plough furrows, has removed any trace of a defence together with any definite evidence for internal structures. Three shallow scoops some 26m to the south west of the inner gateway may however represent hut platforms.

The surviving ramparts flanking the entrance are not immediately visible to those approaching from the steeper slopes to the south and west. These defences also face rising ground to the north and east, and were built in a slight hollow against a band of rock and alongside a spring and shallow valley. These facts, coupled with the lack of a complete defensive circuit to augment the impressive entrance ramparts, probably contributed to the fort's obscurity. No trace of the fort can be seen on nineteenth century Ordnance Survey mapping. It is of interest to note that in October 1981 Ordnance Survey surveyors visited the site of two spurious geological marks identified from vertical air photos which occur on the hillslope only metres to the northwest of the hillfort (Pant yr Ongle enclosures; CPAT PRN: 4631; NAR SN 98 NW 17). Clearly the hillfort was not identified during the same visit, confirming its relative invisibility from the north. A series of excavations cutting into the inner and middle rampart may indicate some form of `archaeological' exploration at some point in the past.

The Pen'r-allt hillfort forms the southernmost example of a coherent group of hillforts sited around the Afon Clwyedog, and which can probably be extended east to include Cefncarnedd near Llandinam. The defensive arrangement of three short ramparts at the apex of a long ridge has much in common with neighbouring hillforts including Cefncarnedd (built on a far larger scale). The form of the innermost rampart, as a 'barrier' with no clear entrance gap, can also be paralleled at other mid-Wales hillforts including those found close to Tregaron in north Ceredigion including Castell, Tregaron and Tre-coll.

Toby Driver & Louise Barker, RCAHMW, 28 November 2006.