Castellau ("fortifications"; alternate: Castella) is a hamlet, with a country house of the same name in the county borough of Rhondda Cynon Taf, South Wales. Historically, it lies within the parish of Llantrisant, just north-west of Beddau. It is connected with the history of the Trahernes. In 1988, Ysgol Castellau became the first Welsh medium education school to open in the southeast within new buildings.