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Bon y Dom

Also known as, or recorded in historical documents as;
The Castle of King Olaf; Porth Bouery Don; Porth Bon y don; Dinas camp

In the community of y Felinheli.
In the historic county of Caernarfonshire.
Modern authority of Gwynedd.
Preserved county of Gwynedd.

OS Map Grid Reference: SH519671
Latitude 53.18067° Longitude -4.21752°

Bon y Dom has been described as a probable Timber Castle.

There are no visible remains.

This site is a scheduled monument protected by law.


The pedigree of Gruffydd on his mother's side. King Gruffydd, son of Ragnaillt the daughter of Olaf, king of the city of Dublin and a fifith part of Ireland and the Isle of Man which was formerly of the kingdom of Britain. Moreover he was kng over many other islands, Denmark, and Galloway and the Rinns, and Aglesey,and Gwynedd where Olaf built a strong castle with its mound and ditch still visible and called "The Castle of King Olaf." In welsh, however, it is called Bon y Dom.
In Leland's Collectanea (ed. Thos. Hearne, Oxford, 1715), p. 85, it appears in the list Havens, Islands, Rivers, etc., as Porth Bouery Don. In Miss L.T. Smith's edition (p. 129 and note) it is written Porth Bon (finis) y don (than a wave) showing that Leland made no mistake between don (=wave) and dom (=mound). It is identified as Moel y Don Ferry (Ibid.). (Jones 1910)

The record of one lost tomen may be concealed in the modern name of the Moelydon ferry, which has been misspelt in various ways. The original form was Bon-y-Dom—“ the stump, or base of the tomen.” The history of Gruffydd ap Cynan says that Gruffydd’s maternal grandfather Olaf King of Dublin "was king over . . . Anglesey, and Gwynedd where Olaf built a strong castle with its mound and ditch still visible and called 'The Castle of King Olaf.’ In Welsh, however, it is called Bon y Dom." This record points to a castle mount built by a man who died in 1034, which had become derelict by the end of the following century. No mound can now be identified on the Anglesey side, nor is a satisfactory one recorded in Caernarvonshire. As the record suggests that the site was in Caernarvonshire, the historical importance of what would have been the earliest motte on record will be dealt with in the Inventory of that county. (RCAHMW 1937).

During the early sixteenth century the old Llanidan ferry, now in Crown hands, began to operate from a new location at Moel y Don, a small promontory on the Anglesey shore, near Llanedwen church. The ferry took a new name to reflect the change ‘Llanidan alias Bon y Don’. A possible explanation for the move might be to do with silting and sandbar formation along the Llanidan reach. At Moel y Don the crossing is shorter and clear and the ferry again became profitable.
The style of the ferry’s name ‘Moel y Don’ is much more frequently used than the rare ‘Bon y Don’. Nevertheless, the name is of interest as it closely resembles the Welsh name of the ‘castle’, built in Gwynedd, on the mainland side, by Olaf the Viking, king of Dublin, Man and the Isles. The name is given in the Welsh biography of Gruffudd ap Cynan (d.1130) which describes the castle as having a mound and ditch: ‘It was called Olaf’s castle. However, in Welsh, it was called Bon y Dom (translated as ‘the bottom-of-the-mound-castle’)’.
It is possible that the re-sited ferry acquired a new name from the prominent topographic feature on the mainland shore near the landing at Felinheli. The landmark is a natural coastal mound, fortified around its base by an earthwork and now known simply as ‘Dinas’. (Gwynedd Archaeological Trust)

Dinas, a cliff fort bordering the Menai Strait near Port Dinorwic. The interior contains a natural ridge which rises to 70ft above OD and then falls away to the Strait in a cliff which forms a natural defence on the north north-west. The landward side is protected by a rampart, in plan almost a semi-circle 390ft in diameter with an entrance near the middle. East of the entrance the rampart has been almost levelled, to the west it survives as an artificial scarp capped by a rubble bank about 12ft wide, giving an external height of 10-20ft. Against the bank are the footings of two rectangular buildings, probably medieval (PRN 3683).
A hoard of Roman coins is said to have been found on the site (RCAHMW 1960).
The site is now densely overgrown. Published survey 24" revised. (Gwynedd Archaeological Trust HER)

The speculation that Bon y Dom is Dinas Camp seems entirely reasonable and, while this can not be entirely proven, Gatehouse has accepted that identification.
Although described as a mound in several reports the fortification of the natural hill at Dinas is not a motte. However it is probable that the castle studies authorities have not identified Dinas as Bon y Dom because they had a fixed idea of what a 'castle' would be.
Question remain. Dinas Camp is identified as pre-historic on its form but it hasn't been excavated. It contains building foots dated as medieval again on form but again not excavated. Is there any substance to the story of King Olaf building castles or is this a back formation to explain older existing forts? If Olaf did build a fortification did he just renew an old fort or did he build a new fort in an old style?
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This record last updated 02/07/2016 09:15:35