A Second World War Seagull Trench set into the north-facing slope of Dinas Dinlle hillfort (NPRN 95309) and protected as a Scheduled Monument (CN396).

Constructed 1940-41 as part of the system of perimeter defences associated with the former RAF station at Llandwrog (NPRN 309961 now Caernarfon Airport), some 2 kilometres to the north.

The Seagull Trench faces north towards RAF Llandwrog and is aligned east-north-east by west-south-west.  It is of concrete and brick construction, partly buried into the hillside with a thick, pre-cast concrete roof. It measures 22.4 metres long, c.4 metres deep and c.2.5 metres tall.

Its name is derived from its shape on plan which resembles a bird. The structure is mostly subterranean with only a concrete roof and ten embrasures facing west, north and east visible externally. The embrasures comprise long thin slit openings and are now nearly all blocked with brick. An entrance was situated to the south on the east side of the structure and is also blocked. It was approached via a short L shaped passage/tunnel which possibly originally led to stairs and a hatch opening. Internally the structure was divided into four wings and a central area by brick blast walls, with the entrance passageway leading into the central area. There was the provision for six weapons within the Seagull Trench, possibly of two different calibres. It is also clear that there were masts or aerials at each end of the structure which presumably linked the defenders in the Seagull Trench to the command system for RAF Llandwrog.

Around the structure are a series of associated earthworks. This includes a scarp slope, sculpted from and enhancing the hillside directly in front of the structure, and continuing to the west for 60 metres. It stands 2.1 metres high in front of the structure and has been disturbed by digging at its eastern end, where two scoops/pits are visible. Running east from the structure are a series of linear earthworks, possibly associated service trenches or an access route to the Trench.

The seagull trench was surveyed using GNSS by the EU-funded CHERISH project in 2017, as part of a wider earthwork/topographic survey of Dinas Dinlle hillfort.

Louise Barker, CHERISH (RCAHMW), May 2023

CHERISH PROJECT. Produced with EU funds through the Ireland Wales Co-operation Programme 2014-2020. https://cherishproject.eu/en/

Sources:

Cadw, 2006 Summary Description of a Scheduled Monument. Dinas Dinlle Seagull Trench CN396 https://cadwpublic-api.azurewebsites.net/reports/sam/FullReport?lang=&id=1168  

Defence of Britain Project (Record Number 11362) https://archaeologydataservice.ac.uk/archives/view/dob/

Spencer, J., 2002 , Three Airfields of the RAF in Northwest Wales: The perception, protection and management of 20th century military remains