As the seat of the fiercely contested lordship and cantref of Maelienydd, Cefnllys became a source of friction between Llywelyn ap Gruffudd and Roger Mortimer in the prelude to Edward I's conquest of Wales (1277–1283).

[2] The castle was captured and slighted in 1262 by Llywelyn ap Gruffudd, Prince of Wales and Gwynedd, during a war with Henry III of England, and Cefnllys featured prominently in the ensuing Treaty of Montgomery.

The construction of a new castle on the south-east side of the hill by Roger Mortimer was a contributing factor to Llywelyn's refusal to swear fealty to Edward I in 1275, leading to war in 1277.

The castle may have been sacked during the revolts of Madog ap Llywelyn (1294–1295) and Owain Glyndŵr (1400–1415), but remained occupied until at least the mid-15th century, when it was described in a series of poems by the bard Lewys Glyn Cothi.

The town was unsuccessful and disappeared altogether as a result of the Black Death and subsequent bubonic plague outbreaks, economic remoteness and changing frontier military conditions, although Cefnllys retained its borough status until the 19th century.

It occupies a key position at the junction of several tributary valleys of the River Ithon, granting commanding views over an important communication corridor into central Wales.

As the English Marcher lords expanded their control westward, Maelienydd formed a core part of the turbulent area known as the middle march, together with Gwrtheyrnion and Elfael.

[21] Prior to the 14th century, sources are limited to accounts of military campaigns and the castles are referenced in the Welsh chronicles Brut y Tywysogion and Annales Cambriae.

Greater stability following Edward I's conquest of Wales resulted in a growth of documentary evidence in the Welsh Marches, though at Cefnllys this is largely restricted to rudimentary public records as the majority of the Mortimer estate archives have been lost.

[22] An extensive topographical survey combined with photogrammetry was carried out at Castle Bank in 1985 by the Royal Commission on the Ancient and Historical Monuments of Wales (RCAHMW), with a follow-up appraisal in 2006.

Indications of a pre-Roman provenance include the ridge's large, elongated enclosure of 10 hectares (25 acres),[23] interpretation of visible earthworks and the extensive use of scarping and ditching to artificially steepen the gradient of slopes surrounding the hilltop.

[27] A silver thumb-ring recovered on Castle Bank may date from an early medieval period of Welsh occupation,[28] and the circular graveyard ringed by yew trees could denote a pre-Norman origin of St Michael's Church.

[13][35] The death of Llywelyn in April 1240 led to an internal power struggle within Gwynedd, presenting Ralph Mortimer II with an opportunity to strengthen his position in Maelienydd.

[note 4] In response, Roger Mortimer levied an army of Marcher lords and arrived at Cefnllys to start repairs on the walls, but was caught off guard when Llywelyn surrounded him with a larger force.

[44] Llywelyn allowed the Marcher force to retreat, a chivalrous gesture probably designed to strengthen his case at future peace negotiations,[13][45] before destroying the remaining defences and continuing his campaign against England.

[53] The historian Robert Rees Davies wrote that Llywelyn, who also faced intense financial and domestic pressure,[54] came to suspect "an orchestrated attempt to undermine his hard-won gains, especially in the middle March, and to subvert the terms of the Treaty of 1267".

[55][56] Cefnllys was not directly attacked in the ensuing conflict but formed part of a chain of garrisoned castles encircling Llywelyn's territories, which contributed to the rebellion's suppression.

[note 5] In 1306, the castle passed into the hands of Roger Mortimer, 1st Earl of March, who led an unsuccessful rebellion against Edward II which resulted in the confiscation of his holdings, including Cefnllys, in 1322.

[63] This may have been insufficient, as a 1405 grant to Richard Grey described the lordship as "burned and wasted by the Welsh rebels",[64] though it is unclear whether the castle itself was sacked or if the destruction was limited to its hinterland.

[12] The castle survived into the 15th century in a reparable state, but its upkeep was increasingly neglected as advances in warfare and shifting social patterns undermined its importance.

The poems, which survive in a manuscript from 1468 (although they probably date from 1432 to 1459), praise the Welshman Ieuan ap Phylip, who was constable of Cefnllys Castle and receiver of the lordship of Maelienydd.

[12] Ieuan had a two-storey hall built to exhibit his status and entertain guests, and, uniquely, Lewys' poetry records the name of the master carpenter as Rhosier ab Owain.

A series of earthworks surrounding the church may represent raised causeways, sunken roads and medieval house platforms,[79] although archaeological finds have been mostly from the Tudor period or later.

[19] Davies comments that its location, accompanying a castle site chosen for defensiveness and ability to radiate strategic power, undermined its economic viability: "the artificiality of [its] commercial setting was too obvious once the military opportunities ... had been removed".

Cefnllys remained a borough after the decline of the medieval settlement,[85] and according to the 1831 census it comprised, besides the church and castle ruins, 16 inhabitants in "three Farm Houses and one small Cottage".