Ynys-y-Castell Promontory Fort is an earthwork enclosure crowning the highest point of a craggy islet at the mouth of Abercastle Bay. It is a roughly level enclosure about 30m square, with its southern and western sides defined by a low L-plan bank, about 30m from east to west and 25m from north to south, with natural scarps elsewhere.
The enclosure is portrayed on the 1st edition Ordnance Survey County series map (Pembroke VIII.11 1889). Its date and function are uncertain. Its position suggests a defensive role, but its enclosure is relatively slight. It does not seem to be a later Prehistoric type enclosure or cliff castle, and could be medieval or later.
A mound south of the enclosure is identified as 'the Grave of Sampson's Finger', although it is likely to be a natural feature (the Biblical Sampson was supposed to have lifted capstones at various cromlechs using only one finger).

John Wiles, RCAHMW, 3 March 2008.

The site is a tidal island approached (at low tide!) by a wide rock causeway. Access to the flat topped summit of the island is by a short cliff and a steep path from the SW side. There are two main areas of interest: The summit has traces of what could be defences, these roughly enclose an area about 30m square. There are two lines of bank - one at the NE from cliff to cliff in a straight line and the other an "L" follows the cliff top at the W and then turns across the island to the cliff at the E. All these banks are very slight and overgrown by a thick blanket of tussock grass. Within this enclosed area is what might be the traces of a hut indentation. It is conceivable other habitations may have existed here. The other main site of the island is what is called "The Grave of Sampson's Finger" (Bedd Bys Sampson). This is a large indentation about 12m x 8m on the gently sloping area to the SW of the summit. In this area are also traces of hut scoops. The "Grave" may simply a larger version of one of these. Some of the remaining evidence could be natural and not man made but on the whole the island exhibits a fair degree of development as a defended settlement, probably of the pre-Roman Iron Age although some writers have seen parallels with Gateholm which has later post-Roman associations. 25m above O.D.

John Latham RCAHMW 06 August 2013 (from NT report "Longhouse", 1993)