Castell Bryn Gwyn is a prehistoric site on the Welsh Isle of Anglesey, west of Brynsiencyn. Its name means "White Hill Castle". The original use of this site is uncertain although it may have been a religious sanctuary. Later Neolithic pottery indicates use in this period, and it may have been a henge monument at this time. The earliest bank and ditch belong to the end of the neolithic period (2500-2000 BC). During the Iron Age the present wall was built, and refortified in Roman times and later. The circular clay and gravel bank, still some 1.5m high, surrounds a level area some 54 metres in diameter, now revetted by stone walls.
Parking is exiguous; the site is accessible from the roadside by footpaths. One path follows the low ridge, southwest over stiles to the Bryn Gwyn Stones, or northeast, past the site of the former stone circle of Tre’r Dryw Bach, some 800 metres to Caer Lêb.