On the steep, narrow north-east facing ridge of Allt Du, between approx.400-450m O.D. are two well-built earthworks that resemble Iron Age promontory defences but which have previously been identified as quarries (CPAT HER, PRNs 34065 and 34556). Both earthworks feature a rock-cut ditch downslope c.6m broad, with a bank or rampart upslope. Both earthworks curve around the contour and feature a simple gateway gap centrally placed on the crest of the ridge. The lower earthwork is 75m long; 144m further up the ridge is the second earthwork of a similar length.

The proximity of Plas y Gaer defended enclosure below (NPRN 305740) increases the likelihood that these earthworks are an unusual form of Iron Age defensive work straddling the nearby ridge which ultimately gives access to the summit of Pen y Fan above. It may be that they protected Plas y Gaer from unwelcome visitors from the south-west, or perhaps performed a ceremonial function on journeys between the lowland defended enclosure and the mountain summit.

The earthworks were recorded during RCAHMW aerial reconnaissance on 23rd February 2016.

T. Driver, RCAHMW, October 2016