Castellau ("fortifications"; alternate: Castella)[1] is a hamlet,[2] with a country house of the same name in the community of Llantrisant, in the county borough of Rhondda Cynon Taf, South Wales.

[4] A forge was situated in a small valley, Darren Ddeusant,[5] extending upwards by the mansion, Castellau House, into the hills to the north of Llantrisant.

The interior has a semicircular staircase, square hall, bow-ended dining room, Ionic columns, marble chimneypiece, cast iron balustrade.

The white house's exterior features bowed ends, a six-bay facade, central doorway, slate roof, cantilevered eaves, and a cast iron veranda.

[11] The chapel is located 1 mile (1.6 km) south of the house; featuring angled pinnacles and transomed lancets, the interior has galleries on three sides and a polygonal pulpit.