There have been claims that this was a windmill, but Gerallt Nash of the Welsh Mills Society informs me
You have St Justinian's tower, nr St Davids, Pembs, noted as a windmill.
I'm afraid it isn't (even though the local National Trust office/shop has a photo of it captioned as
former windmill). It is/was in fact a folly or possibly small summer house or 'look-out/viewing point'
for the large bungalow in whose grounds it stands and which overlooks the dramatic Ramsey Sound
and Ramsey Island about 1-2 miles west of St Davids.