The castle at Bronllys is sited above the floodplain of Afon Llynfi and is thought to occupy the site of a pre-Norman llys, a princely court. A fire involving a stone falling from the 'principal tower' was recorded in 1165 and the castle was prepared against Owain Glyndwr in the early fifteenth century.

The surviving remains consist of an irregularly pentagonal enclosure about 156m north-south by 136m defined by sections of banks and ditches. A curving or polygonal ditched bailey occupies the southern half of the enclosure. A print of 1741 shows the ruins of a large two-three storey building in this area, part of which are thought to survive within modern buildings (see NPRN 31124). The great round tower is probably early to mid thirteenth century (with windows of the fourteenth century), and is set upon a ditched mound at the enclosure's southern apex. This probably replaced a twelfth century structure. A late eighteenth-nineteenth century house (NPRN 25159) stands within the enclosure, which has been landscaped as gardens and grounds (NPRN 86020).

RCAHMW, December 2008.