The remains of Llanthomas Castle Mound consists of the motte, the ditch and buried masonry underpinning part of the wooden fence surrounding the bailey.

[9] Antiquarian, Tudor, Edwardian, Victorian and modern sources identify Llanthomas Castle Mound with names reflecting its close proximity to Llanigon and Hay-on-Wye.

[14] The Ministry of Works published a list of UK monuments whose preservation was considered to be of “national importance”.

[26] Llanthomas Castle Mound is near the village of Llanigon[27] and less than 2 miles from the "town of books" i.e. Hay-on-Wye.

[30] Llanthomas Castle Mound is in the foothills of Hay Bluff in the Brecon Beacons (Bannau Brycheiniog).

[31] The location of Llanthomas Castle Mound may have been chosen because it occupies a high point that once overlooked the River Wye less than a mile away.

The fording point Little Fford Fawr[33] is located between Llanthomas Castle Mound[24] and the south bank of the river.

The report concludes that Llanthomas Castle Mound is an important relic of the medieval landscape.

The Clwyd–Powys Archaeological Trust(CPAT) records for Llanthomas Castle Mound include past Cadw reports: PRN 443 (1986),[40] 38278 (1988),[41] 2586 (1995).

The archive record for Llanthomas Castle Mound (PRN 306308 [43]) include a hundred years of reports: 6057064, 6054097, 6064626, 6140925, 6140927, 6359576, 6464877, 6140926, 6140924, 6054098, 6059886, 6519900.

In 1921, the Reverend William Edward Thomas Morgan vicar at the pre-conquest church of St. Eigon, Llanigon,[44][45] an amateur archaeologist hosted a visit from the Woolhope club.

[46] There is no known record of any professional level archaeological excavation or geophysical survey of Llanthomas Castle Mound.

A number of artefacts were found including a sherd from the base of a medieval cooking pot, and a charcoal filled pit.

Some antiquarian scholars believed that Llanthomas Castle Mound was built on an Iron Age tumulus.

It is not known who built Llanthomas Castle Mound but it is known to have existed from the early days of the Norman conquest.

[61][62][63] The manor included a proprietary church called Thomaschurch, possibly the same chapel of ease, as its name is a translation from the French.