The castle was a text book motte and bailey (lat/long ￼52.462459, -4.028706), presumably of timber that was built by a Norman invader (Walter de Bec), captured by a Welsh chieftain (Owain Gwynedd), retaken by the French, and abandoned, all between 1100CE and 1200CE. On the ground, the earthworks are very evident still, but hard to capture from ground level through spring foliage.Coflein entry has many excellent pictures, some aerial, that make the matter clear.
The castle overshadows the small church of St Michael which is very grand: in its grounds lie a 2000 year old yew and a terrific cemetery on a precipitous incline.
The excellently maintained Llanfihangel Genau'r Glyn website is full of historical information, including a complete list of the cemetery inscriptions, a Herculean achievement. The area was once central to coastal trade, and the parish of disproportionate local importance, meaning that many of the graves are of great interest and significance.
Dr Beeching did for the railway station, but the line remains, with regular trains to Aberystwyth.