The remains of the other candidate for such work, the high-status early mediaeval site, on and near the mound known as the Mŵd, do not now have masonry of broken stone and may not have been large enough to justify the quantities of material recorded.

The modern Garth Celyn is defined by the Royal Commission on the Ancient and Historical Monuments of Wales (RCAHMW) as a series of features, including the house Pen y Bryn itself, a large 'barn', and earthen terraces and scarps, collectively interpreted as an enclosure 90–100 metres across.

[8][9] The Trust identifies modern Garth Celyn as the site of the royal llys (palace) in the 13th century and the centre of government before the Conquest, and suggests that many notable events in Welsh history occurred here.

In 1537 John Leland reported "The Moode, in the paroche of Aber otherwise Llan Boduan, wher Tussog Lluelin uab Gerwerde Trundon (sic) had a castel or palace on a hille by the Chirch, wherof yet parte stondith.

"[15] A high-status structure, associated with 13th and 14th century material and thought to be the royal llys with its domestic and administrative buildings, has been found in the valley bottom, on the other side of the river and some 185 metres from Pen y Bryn.