Castell Mawr Chain Home High Radar site (AMES Number 67) was one of a network of coastal early warning stations built by the Royal Air Force, before and during the early years of the Second World War to detect and track enemy aircraft and provide an early warning capability.

'Chain Home was one of the first practical radar systems, and the main component of the world's first integrated air defence system, the Dowding System. CH radars stretched across the eastern and southern shoreline of the British Isles, offering almost continuous coverage of the over-water areas offshore. CH systems would often detect larger formations of aircraft over France, offering invaluable early warning of an impending raid. This allowed the RAF to use its fighters far more effectively.' (Wikipedia entry accessed 15/06/2016)

Plans are held for the Castell Mawr site by the Public Record Office (ref: AIR 20/7584, dated Oct 1941, marked SECRET). This shows 6 bunkers constructed along the foot of the coastal slope between Maesfron and Plas Allt-Lwyd houses in the south-west to Allt-lwyd-fach farm in the north-east, below Castell Mawr hillfort. These supported two paired mast arrays in fields to the west of Allt-llwyd-fach. Additional structures include 'Q' blocks built by the RAF as decoys to the more concealed bunkers. A pair of these, now used as agricultural barns, survive by the main coast road at SN 529 687. The plan also shows the position of houses for RAF and WAAF quarters within Llanrhystud village.

Recorded during RCAHMW aerial reconnaissance. Information supplied by Roger Thomas.

T. Driver, RCAHMW, 16 June 2016