The base of the Roman tower at Caer-y-Twr has been left exposed and consolidated for public access on the summit of Holyhead Mountain. It lies next to a tall Ordnance Survey cairn and pillar, and a little way from a modern drystone shelter. In 1776 Pennant described a strongly cemented circular building 'lately much demolished by ... vulgar ... persons', and suggested that it was a Roman pharos or lighthouse. Excavations in 1980-1 revealed traces of Roman buildings, including the footings of a 5.45m square stone tower (Crew 1980; 1981), associated with second to fourth century pottery and coins, including three of Constantinian date. It was interpreted as a fourth century watch-tower with a construction date after 340 and a period of use extending into the early AD 390s.

Some 20m to the south-south-west are the tumbled ruins of a roughly D-shaped drystone shelter. The bar of the D includes a 1.5m section of some finer walling standing three courses high. This was mortared according to earlier accounts (Willoughby Gardner 1934, 172). There is no trace remaining of a second cairn recorded by RCAHMW on the summit area some 17m south-east of the tower (Inventory, 23).

An ostensibly similar tower has been excavated at Capel Eithin (NPRN 43559) and a less certain example has been noted at Pen Bryn-yr-Eglwys (NPRN 43547), both on Anglesey. Elsewhere, a tower stands at the highest point of the Dinas Emrys fort (NPRN 95284).

Sources: Willoughby Gardner in Archaeologia Cambrensis 89 (1934), 156-73.
RCAHM Anglesey Inventory (1937), 23
Lloyd in Archaeologia Cambrensis 113 (1964), 150-158
Crew in Archaeology in Wales 20 (1980), 42-44
21 (1981), 35-6

John Wiles, RCAHMW, 27 April 2007