Ancient Monuments

History on the Ground

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Castell Bryn-Gwyn

A Scheduled Monument in Llanidan, Isle of Anglesey (Ynys Môn)

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Latitude: 53.1784 / 53°10'42"N

Longitude: -4.298 / 4°17'52"W

OS Eastings: 246523

OS Northings: 367052

OS Grid: SH465670

Mapcode National: GBR 5H.3LG7

Mapcode Global: WH436.YCJQ

Entry Name: Castell Bryn-Gwyn

Scheduled Date:

Source: Cadw

Source ID: 2511

Cadw Legacy ID: AN015

Schedule Class: Religious, Ritual and Funerary

Category: Henge

Period: Prehistoric

County: Isle of Anglesey (Ynys Môn)

Community: Llanidan


The monument consists of a near circular earthwork enclosure defined by a massive rampart that remains up to 2.6m high, with an internal area in the region of 52-56m across. There is little trace on the ground of a ditch and early observers, misled by irregularities in the rampart, identified this as a Roman amphitheatre.

Excavations in 1959-60 demonstrated that the monument had originated as a late Neolithic ritual henge enclosure or similar. It had later been adapted as a defensible circuit, presumably enclosing a settlement. This reuse probably occurred in the later Prehistoric period and its latest phase is associated with Roman pottery of the late first century AD.

The primary Neolithic phase had a 5.2m wide stony bank with a 2.3m wide berm separating it from a broad flat-bottomed external ditch, 1.9m wide and up to 9.8m wide. There was at least one entrance, facing south-west. The ditch had partly silted up and the bank had weathered, when the bank was extended forward into a 9.0m wide rampart, revetted by a dry stone wall and fronted by a relatively insubstantial ditch. Finally the rampart was again extended to a width of 11m with a timber revetment and a new ditch was dug, 5.5m wide and 3.6m deep. The south-west entrance was blocked at this time and a Roman pottery sherd was recovered from the blocking.

The site has been associated with the castle called Bon y Dom built by Olaf in the 11th Century, although nothing has been proven. It was excavated by G J Wainwright in 1959-60. The north east section of the bank has been removed and overlain by farm buildings, and the south east section is overlain by the farm track, although traces of the bank are still visible either side of the track.

The monument is of national importance for its potential to enhance our knowledge of prehistoric burial and ritual practices, and later prehistoric defensive organisation and settlement. It retains significant archaeological potential, with a strong probability of the presence of associated archaeological features and deposits. The structure itself may be expected to contain archaeological information concerning chronology and building techniques, together with a strong probability of environmental evidence.

The scheduled area comprises the remains described and areas around them within which related evidence may be expected to survive.

Source: Cadw

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