Llanidan is a community in the south of Anglesey, Wales which includes the village of Brynsiencyn (Welsh pronunciationⓘ).

On the basis of field names it has been suggested that the Roman army under Suetonius Paulinus landed here in 60, and again in 78 under Agricola, overcoming the Ordovices of north-west Wales and Anglesey, at a spot known as Bryn Beddau, (Hill of Graves in Welsh).

[2][3] In 1867 it was suggested that the local field names "Maes Hir Gad" (Area of long battle) and "Cae Oer Waedd" (Field of cold or bitter lamentation) may indicate the site of these battles [4] Near Tai Cochion house, excavation and geophysical survey has revealed part of a Roman settlement of unusual layout, and on the opposite side of the Menai Strait to the Roman fort at Segontium.

Many of the enclosures contained fairly clear, rectangular anomalies interpreted as buildings, with typical dimensions of around 16m x 8m.

The layout, with a central road flanked by plots containing rectangular buildings, is similar to Roman villages and small towns such as Sedgefield.

This arrangement has no defences and is otherwise unlike the military Roman sites in Wales, nor does it resemble the native defended settlements with their roundhouses and irregular outlines, nor the occasional villas; it suggests a previously unknown level of civilian Romanization in the remote west of the province.

[11] Geophysical survey at the western end of the site shows a large prehistoric defended settlement.