Tomen Las is a medieval castle represented by a mound or motte, that would originally have been crowned by a great timber framed tower. The castle is undocumented, however, it was probably attached to the court or llys of the Princes of Gwenydd at Pennal, removed from the earlier site in the Roman fort on Cefn Gaer (see NPRN 300159). A princely hall and associated offices would have stood near the mound, although the site is generally marshy.

The surviving remains include a circular mound 26m in diameter that rises 3.0m from the traces of its ditch. The level summit is 15-17m across. There are no traces of further earthworks. The mound was treated as a tree clump in Plas Talgarth park (see NPRN 265177).

Source: History of Merioneth II (2001), 412

John Wiles, RCAHMW, 11 February 2008