Though all that remains of this Neolithic (New Stone Age) settlement is a single bank, excavations have revealed a site with a particularly long history. Its origins stretch back to a circular enclosure featuring a bank and external ditch built in the late Neolithic era or early Bronze Age. Originally thought to be a ceremonial henge monument, finds of pottery, post holes and flint and bronze tools are in fact those of a settlement.
The locals must have liked the place. Castell Bryn Gwyn was used far beyond the Neolithic period, with archaeological finds suggesting it was still inhabited as late as 1st-century Roman times.
https://cadw.gov.wales/visit/places-to-visit/castell-bryn-gwyn