Caer-y-Twr is the remains of an Iron Age hill fort on the summit of Holyhead mountain (Mynydd Twr) 220 metres in height. Due to its position, it did not need much additional defence, but it had a stone rampart on the northern and eastern sides enclosing an area of roughly seven hectares. The site of the hill fort now contains mostly rubble, but the walls can still be identified.
There are also the foundations of a Roman tower on the mountain, which date from between 100 and 300AD. The exact use of the tower is uncertain; some people think it was a watch tower, others a signal tower or even a lighthouse. The site is now managed by CADW.