Castle-uopn-Alun House is a long two-storey range with rear wing and has rough cast over masonry under steeply pitched slate roofs. The central unit of the house shows its sub-medieval origins. It is shown on the St. Brides Major Tithe Map of 1840 when it belonged to John Nicholl of the Merthyr Mawr Estate. The house was substantially remodelled and extended c 1850 and further remodelled during the early twentieth century. The fenestration is generally 12-pane horned sash windows with flat heads and stone sills.
Attached rubble walls define the garden to the south and east of the house. The Gothic doorway near the SW angle of the house probably reuses medieval stonework and there is a similar Gothic doorway at the SE angle of garden walls. A square stone outbuilding is also incorporated on the east side.
It is listed as a well preserved example of a gentry house of several periods whilst the retention of the extensive garden walls is of additional interest. The sub-medieval origins of the house provide additional historic interest. It has group value with the adjacent stables and cartshed.

Source:- Cadw Listed Buildings, NJR 06/07/2010