Joseph Nelson was the engineer and builder of Bardsey Light, but the design carries the stylistic fingerprints of Daniel Alexander, who had succeeded Samuel Wyatt as consulting engineer to Trinity House and under whom Nelson served. You can see Alexander's influence in the heavy weathered string-course near the base and in the curiously blocked and hooded directional-light window. Why square? Nobody is entirely sure. The only other British lighthouse of the period built in this form is Coquet Lighthouse off Northumberland, built in 1841 and designed by James Walker for Trinity House. Salt Island Lighthouse at Holyhead - also Rennie, also 1821 - shares some of the same distinctive iron gallery railings, bellied outwards at the crown. It was a brief moment in British lighthouse design. The round-tower won. The square towers stayed where they were and have been working ever since.

The tower is built of ashlar limestone, unplastered inside and out, painted in those distinctive red and white bands on the outside. It is 30 metres high - about 100 feet. The plinth at ground level forms a square 7.6 metres across, narrowing to 6.1 metres at the top of the plinth, and 4.6 metres at the top of the tower below a heavy projecting cornice that juts out again to 5.5 metres. The walls are 1.2 metres thick at the base, tapering to under 0.9 metres at the top. The original 1821 light used reflectors. In 1838 it was upgraded to a dioptric refracting mechanism. The current lantern - fitted in 1856, a 4.27-metre wide chamfered octagon - did not initially revolve; the present revolving apparatus was installed in 1873 and produces a group of five flashes. The original oil vapour lamp was replaced with electric in 1973. Trinity House finished a comprehensive refit in 2014. The light has been continuously in service for more than two centuries.

There is one persistent problem. Bardsey sits squarely on a major migratory bird route, and lighthouses kill birds. The bright light at night confuses them. They fly toward it. They hit the tower or the glazing. Trinity House and the Royal Society for the Protection of Birds have tried various interventions over the years - perches for tired birds to land on at the lantern level, flood-lighting the exterior of the tower so it is visible as a structure rather than as a bare lure of light. The interventions have not really helped. The casualties continue. It is a quiet, melancholy footnote to the history of any coastal light in a major bird flyway: the same beacon that saves sailors costs the lives of thousands of warblers and shearwaters every year, and there is nothing simple to do about it.

One more peculiarity. Bardsey Lighthouse is unusual in lacking any sort of harbour or quay facilities at its base. Most British lighthouses, especially the offshore ones, have some kind of small dock or hoisting equipment that allowed Trinity House launches to deliver supplies and relieve keepers. Bardsey was reached by boat to the island's only landing place at Y Cafn - a small notched cove near the northern end - and then by tracked vehicle or foot along the entire length of the island. Y Storws, the storehouse near Y Cafn, was built a few years before the lighthouse itself to receive supplies. When the keepers were on station, this is how they were resupplied: by boat to Y Cafn, then a mile-and-a-half overland to the southern point. Automation has made that journey unnecessary now. The light works on its own.