To inform the consideration of The Inventory of Historic Battlefields in Wales , a phased programme of investigation was undertaken on the battle of Bryn Derwin. Detailed reports of these investigations are available and comprise documentary and historical research (Chapman), and non-invasive and invasive fieldwork (Archaeology Wales).

Bryn Derwin was a battle for the unity and independence of Gwynedd, born out of the division and dissension that followed the deaths of Llywelyn ab Iorwerth (d. 1240) and Gruffudd ap Llywelyn (d. 1246).

The basic outline of the battle is given in the Red Book of Hergest version of Brut y Tywysogion for the year 1255:

'Y dydyeu hynny y magwyt teruysc mawr o annoc kythreul rwg meibon Gruffud ap Llywelin, nyt amgen, Ywein Goch a Dauid o'r neill tu, a Llywelin o'r tu arall. Ac yna yd arhoes Llywelin a?e wyr yn diofyn ym Bryn Derwyn, trwy ymdiret y Duw, creulawn dyuodyat y vrodtr, a diruawr lu gantunt. A chynn penn vnawr y delit Ywein Coch ac y foes Dauit, wedi llad llawer o?e lu a dala ereill a ffo y dryll arall. Ac yna y caracharwyt Ywein, ac y goresgynnawd Llywelin gyuoeth Ywin a Dauid hep wrthwynep idaw' (Jones, 246).

Translation: 'In those days great strife was bred at the instigation of the Devil between the sons of Gruffudd ap Llywelyn, namely, Owain Goch and Dafydd, on the one side, and Llywelyn, on the other side. And then Llywelyn and his men, trusting in God, awaited unafraid on Bryn Derwin the fierce coming of his brothers, and a mighty host along with them. And before the end of one hour Owain Goch [Owain ap Gruffudd] was captured and Dafydd fled, after many of his host had been slain and others had been captured and the remainder had fled. And then Owain was imprisoned, and Llywelyn gained possession of the territory of Owain and Dafydd without opposition to him' (Jones, 247).

However, this version is alone in suggesting that Dafydd fled. Other accounts, such as the Breviate Chronicle (Remfry, 194) and the Peniarth 20 version of the Brut y Tywysogion (Jones, 110) indicate that he was captured.

The clearest evidence concerning the site of the battle comes from a praise poem directed toward Llywelyn by Llygad Gwr which includes the following lines:

Yng nghymysg Arfon ac Eiddionydd.

Ei dreisiaw ger Drws Daufynydd (Gruffydd)

Translation: On the borders of Arfon and Eifionydd,

To vanquish him by Drws Daufynydd (Smith, 72)

This has been interpreted as being near the modern Bwlch Derwyn and Derwyn Fawr (SH 464 461), on the borders of the parishes of Clynnog and Dolbenmaen, just east of Bwlch Dau Fynydd (the pass of the two mountains ? Drws Daufynydd/the door of the two mountains) (Lloyd, 715). Fieldwork in this area undertaken in 2014 did not find evidence that could be linked to the battle (Archaeology Wales).

RCAHMW (Battlefields Inventory), Jan 2017

Bibliography
Archaeology Wales, 1255 Battle of Bryn Derwin: Battlefield Survey (2014).
Chapman, A., Bryn Derwin 1255: Documentary and Historical Research Report (2013).
Gruffydd, R. Geraint, Gwaith Bleddyn Fardd ac Eraill (University of Wales Press, 1996).
Jones, Thomas (trans.), Brut y Tywysogyon or The Chronicle of the Princes Peniarth MS. 20 Version (Cardiff, University of Wales Press, 1952).
Jones, Thomas (ed. and trans), Brut y Tywysogyon or the Chronicle of the Princes: Red Book of Hergest Version (Cardiff, University of Wales Press, 1955)
Smith, J. Beverley, Llywelyn ap Gruffudd: Prince of Wales (University of Wales Press, Cardiff, 2014).