Holyhead Mountain (Mynydd Twr in Welsh: from (pen)twr, meaning "tower") is the highest hill on Holy Island, Anglesey, and the highest in the county of Anglesey, north Wales. It lies about two miles west of the town of Holyhead, and slopes steeply down to the Irish Sea on two sides. There is a late Roman watchtower, called Caer y Tŵr (Tower Fortress) from which the Welsh name is derived on the east side of the hill.