Castell Trefilan is a strong castle mound that can be associated with a princely house and may be part of a suggested borough (NPRN 401517). It is a rather oval flat-topped and moated mound, about 40m north-south by 35m, and 5.4m high. The oval summit, currently 15m north to south by 12m, is shown as about 21m by 18m on OS County series 1st edition, banked about and enclosing an area approximately 12m across. The moat, generally 60m overall diameter, is thought to have originally held water. Explorations on the motte summit encountered a line of rounded boulders 'like the foundations of a wall' (King 1959, 66). The was reputedly established in 1233 and may be associated with the 'house' at Trefilan burnt in 1282.

The trees upon the mound were felled in 2003. Royal Commission aerial reconnaissance in confirmed the presence of the original outer bailey as germination marks or early cropmarks in the cultivated field to the south-west. The marks show a ditch c.5m wide running west in a shallow arc, with the terminals of a gateway visible just to the west of the present road. The gate thus lies some 75m south-west of the surviving motte. A projection of the original perimeter of the bailey from these cropmarks suggest an enclosure c. 87m wide x 123m long, with the motte standing at its north-east end.

Sources include:
King in Ceredigion 3 (1959), 50-69;
Griffiths in the Welsh Historical Review 3.2 (1966), 125.

RCAHMW, 25 February 2014