This fish trap is rectilinear in shape and consists of two 'L' shaped walls with Ynys Castell at its south-eastern end. The first 'L' shaped wall consists of low stone walling set with wooden posts, and is depicted on OS maps from the first edition (1890) onwards. It extends south, some 150m from the shoreline, at which point it turns ninety degrees and extends north-east for a further 80m, terminating at Ynys Castell. The second 'L' shaped wall is located some 45m further east and is depicted on an historic Admiralty chart. It also extends south from the shoreline until it reaches the north-east arm of the first 'L'. It then turns ninety degrees and runs north-east for some 30m before turning ninety degrees again and running north for a distance of a few meters. The two walls may represent different phasing. It is probable that the fish trap pre-dates the adjacent causeway leading from the shore to Ynys Castell. If this is the case the fish trap probably worked by chanelling fish between the island and the higher ground adjacent to the shoreline.

Sources include:
Chart 1174-A4 date 1839
Gwynedd Archaeological Trust, 2012, Historic Environment Record
Ordnance Survey, 1890, 1st edition Landmark
Ordnance Survey, 1900, 2nd edition Landmark
Ordnance Survey, 1914-25, 3rd edition Landmark

Maritime Officer, RCAHMW, January 2012.