Pen-y-Castell hillfort occupies a small knoll towards the head of a west-facing valley, with steep slopes on all sides but the east. The hillfort is oval and measures about 110m east-west by 60m north-south. On the north side, the defences comprise a well-built rampart wall with a discontinuous external ditch. The rampart on this side is broken by three main gaps with the corresponding ditch sections in-filled, but these appear to be caused by erosion. The southern circuit of the fort rampart stands tallest at the west and east ends; in the centre the natural slope is steeper and the rampart turns into a low scarp bank. Frequent erosion scars and a very thin turf cover on the remainder of the rampart clearly shows that it was originally entirely stone faced on the outside, with an inner stone revetment also traceable on the north side of the fort. In places the rampart face was walled with even courses of stone, in other places, particularly on the external north face, much larger boulders were utilised.

The single gate is on the east side facing the easiest approach. It is defined by a pair of terminals, the one on the south side being broader and asymmetric and possibly indicating the presence of a bastion. The construction of the gateway is disproportionately grand when compared to the remainder of the defensive circuit. It is flanked by a deep, rock cut ditch but only in two limited sections to the north and south of the gate. The external revetment is of small, dressed stone blocks and is completed to a far higher standard than rougher stone revetment found on the west side of the fort, away from the main gate. Crucially, the rampart wall immediately north of the main gate preserves a narrow berm between it and the edge of the rock cut ditch, effectively indicating little or no erosion or change to the profile of the rampart face since it was originally built. A slight counterscarp on the outer lip of the northern ditch indicates periodic cleaning-out.

The interior of this small hillfort is remarkably well preserved, despite a brief episode of historic ploughing which has left a series of plough-ridges overlying earlier features. These earlier features chiefly comprise the remains of up to six house platforms, first mapped by the Ceredigion Archaeological Survey. The slight remains of two survive in the western half of the fort, on the highest part. In a sunken, more sheltered, area on the south side are the remains of three house platforms, although not necessarily contemporaneous. The best preserved house platform survives in the north part of the fort, directly in from the gate, is circular. A small mound just inside the gate, plus two earth-fast boulders, one in the southern part of the fort and one in the centre of the gateway, may be original Iron Age features of unknown function.

The particular position and orientation of the fort is worthy of note. The hillfort is highly conspicuous on its knoll and directly overlooked from higher ground, particularly from the ridge route to the south. The main gate avoids the possibility of a westward vista towards the coast, and the hillfort is instead orientated to face high ground and the mountain passes to the east.

T. Driver. July 2005.