Caer y Twr is the site of an Iron Age hillfort on the summit of Holyhead Mountain (Welsh: Mynydd Twr). It has a commanding position over the Irish Sea and the remains of a large stone rampart on the east and north sides. There is also the remains of the base of a subsequent Roman stone watchtower. The site is west of Holyhead (Caergybi) on Holy Island (Welsh: Ynys Gybi) the island next to the larger one of Anglesey, Wales (Welsh: Ynys Môn, Cymru). A path can be taken near South Stack to the summit of Holyhead Mountain.
Link: Cadw - Caer y Twr image
Image: Caer y Twr image courtesy of Cadw