A native Welsh castle built around 1260, Castell Du consists of ruinous walls and the fragmentary remains of a mural tower. A perimeter wall encloses an area about 26 metres on one side. In the wall is a projecting tower in the D-shape favoured by Welsh military architects. The tower is 8.5 metres wide, with walls 1.8 metres thick. During World war II a pair of pillboxes were constructed and carefully camoflaged within the castle ruins.
We do not know for certain who was responsible for building Castell Du, but one possibility is Llewelyn ab Gruffudd. An intriguing historical record from 1265 mentions that Edward I destroyed 'a castle beyond Brecon', which may be Castle Du. Note that there are several other fortifications in wales known as Castle Du, for example, there is one near Dolgran in Carmarthenshire.