Caer y Twr (Welsh: Tower Fortress) was a Roman-era hillfort atop the summit of the Holyhead Mountain in Anglesey, Wales. The hillfort, which is situated among rocky outcrops, is ideally placed for defence and likely served as a watchtower and possibly as a signal tower. Some have speculated that it was built to alert a small fort situated in the town of Holyhead in the case of raiders coming in from the Irish Sea, while others have suggested that it may have been a lighthouse.