A vast 9.8ha hillfort crowns the summit of Foel Fenlli, a mountain peak overlooking the vale of Clwyd. The fort is rather irregular in shape, roughly 480m east-west by 74-280m. Its eastern end perches on the actual summit and the remainder is draped across the gentle upper western slopes below. It is enclosed by two or three lines of great ramparts and ditches. These run above headlong slopes except on the east. The ramparts would originally have been crowned by elaborate timber-framed breastworks. There was a single west-facing entrance, an elaborate in-turned structure with outworks.

The interior is irregular. A quarry ditch runs around within the inner rampart. The ultimate summit at the east end shows a 16m diameter round cairn and there is a spring in the hollow below on the west. As many as 40 circular building platforms have been noted by CPAT. These would have carried great thatched roundhouses. A Bronze Age burial cairn crowns the highest summit.
A hoard of at least 1,500 late Roman coins was found following a fire in 1816 and more were found in 1847 (see Archaeologia Cambrensis II (1847), 108-111). 'Diggings' in 1849/50 produced some Roman pottery, including distinctive samian fabric.
The interior contains a large number of hut platforms (NPRNs 501244, 501245, 501247-501262). On the summit, at the east end, is a cairn (NPRN 501246).
Hillforts such as this are characteristic of later Prehistoric settlement and often remained in use through the Roman period.

Sources: Ffoulkes in Archaeologia Cambrensis New Series I (1850), 81-9
Davies 'The Prehistoric & Roman Remains of Denbighshire' (1929), 183-6, 228
I Brown, 2004, Discovering a Welsh landscape: Archaeology in the Clwydian Range, 69.
RCAHMW and H & H, 15 September 2008.