The rise of Christianity in the 6th century was accompanied by the building of monasteries in remote places. Two were built at opposite ends of Anglesey.
St Cybi’s monastery was built at Holyhead or Caergybi in Welsh (fort of St Cybi), and the other on the north-eastern side, called Penmon (the head of Anglesey). The two saints were friends, walking the breadth of the island for a weekly meeting.
Penmon has a range of different and interesting buildings. The oldest is probably St Seiriol’s well, thought to have healing powers, and visited by pilgrims.
The monastery developed over the ages, from a wooden structure, which was destroyed and ransacked by Viking intruders in 971 AD, and then gradually rebuilt as a stone building, throughout the 12th century by Gruffydd ap Cynan and Owain Gwynedd. It is an impressive building, and the best example of its kind in the area.
It is a familiar cruciform design, but during the reign of Llywellyn ap Iorwerth, a refectory was added, including a dining hall, dormitory and cellars.
Henry VIII dissolved the monastery in 1537, and the lands were passed on to the local landowners, the Bulkeley family, and used as a deer park.
Nearby, across the treacherous sound is Puffin Island, or Ynys Seiriol (Seiriol’s Island). A small community was established here at St Seiriol, and a number of mediaeval monastic buildings are to be found here.