Castell Mawr is generally considered to be a later Prehistoric settlement enclosure, possibly of two phases, although it has been suggested that it is an earlier ritual or ceremonial henge enclosure reused in the Iron Age. The site was subject to partial geophysical survey in 1988.

The monument occupies the gently rounded summit of a hill. It consists of a 1.3ha oval enclosure defined by: a slight inner bank; a broad and shallow ditch; a prominent outer bank, preserved as a hedgerow and apparently ditched. There entrances on the north-west and east. The interior is subdivided by a curving west-facing rampart and ditch cutting off the 0.7ha eastern part of the enclosure. No entrance between the two divisions has been identified.

The character of the main enclosure, with a strong outer bank overshaddowing the weaker inner bank, has prompted the suggestion that it represents a Neolithic henge. In support of this flints have been found within the enclosure. However, the prominence of the outer bank may be a product of its reuse as a hedgebank and flints continued to be used into the historic period.
A tanged iron spearhead was found in the field to the south.

Sources: Mytum and Webster 'Geophysical Surveys at Defended Enclosures ...' (2003) - unpublished report
Driver 'Pembrokeshire: Historic Landscapes from the Air' (2007), fig 69

John Wiles 20.02.08

Survey, geophysical prospection and excavation at the site in 2012 by Prof. Mike Parker-Pearson was linked to the possible Neolithic origins of the hillfort/henge and ongoing work at Craig Rhosyfelin quarry (NPRN 416247).

T. Driver, RCAHMW