The earthwork enclosure of Castell Cwm-wyntyll occupies a saddle between two hills, 175m above sea level and 450m north east of Cwm-wyntyll farmhouse in the parish of Trecwm (formerly Llanfair-nant-y-gof), Pembrokeshire.

The earthwork remains define an inner circular area 39m in diameter (0.1 hectares). Later enclosure obscures the complete plan of the monument, with the best preserved sections to the north and west, comprising double concentric banks with an internal ditch. The inner bank is 6.7m wide and 1.3m high, with the outer bank, 3m beyond, 7m wide and 1.4m high. The inner face of the outer bank has a clear stepped profile incorporating a 1m wide terrace. The ditch between the two banks comprises very marshy ground and is largely in-filled with evidence that it has been re-cut in recent times. Despite later alterations, the line and plan of the inner bank has been fossilised in the field boundaries that overlie the eastern and southern edge of the enclosure. There is no surviving evidence for the continuation of the outer bank, if it did ever continue, or for an outer ditch.

The position of entrances into the enclosure is unclear. The two obvious gaps are not original, and the most likely candidate is at the south western corner of the site. The interior also appears modified, much of it looks to have been dug out and now comprises marshy ground covered in reeds. The only feature of significance is in the south-east corner at the junction of two field banks, where there are signs that a later structure once stood here.

The monument is best classified as a small defended settlement, perhaps a protected farmstead, most likely Iron Age in date. This is a monument type seen throughout Wales, with over 220 known in Pembrokeshire alone. Like many such sites in Pembrokeshire it is not sited in an overwhelmingly defensive position, with two higher summits to either side and also exposed to the prevailing wind from the southwest. However other factors, including a good water supply and fertile land may have factored into the choice of location. The obvious presence of water at this site, strongly suggests this was the fundamental reason behind its location, whether for agricultural or even possibly more ritual purposes.

A detailed surevy of the monument was carried out by RCAHMW in March 2010

Louise Barker, RCAHMW, October 2010.