A large and massively walled hillfort crowns the summit of Holyhead Mountain. The wall was cleared of debris in 1913 when the fort was taken into state care.

The fort enclosure has an area of about 6.8ha. The wall runs about the northern half whilst the southern part rests on crags and headlong slopes. Where best preserved the wall is 4.3m wide at the base and stands up to 3.3m high. It is battered on both sides and has a step or walkway on the inner side. The sole entrance is set within a rocky gully, continued within the fort as an avenue flanked by lines of outcrops. This opens onto a broad level area beneath the final ascent to the summit.
Excavations on the highest point of the interior revealed the footings of the eponymous tower (NPRN 308080). This was associated with second to fourth century Roman material. There are traces of a second building nearby.

Sources: Willoughby Gardner in Archaeologia Cambrensis 89 (1934), 156-73
RCAHMW Anglesey Inventory (1937), 24-5

John Wiles, RCAHMW, 27 April 2007