The Brutiau for the year 1153 notes:

Meredud a Rys, meibon Gruffud ap Rys, y bydinoed y Penwedic; ac ymlad a wnaethant a Chastell Howel a?e darestwg (Jones, 130)

Translation: Maredudd and Rhys, sons of Gruffudd ap Rhys, directed their armies into Penweddig; and they laid siege to Hywel's Castle and subdued it (Jones, 131)

The reference to Castell Hywel in the commote of Penweddig identifies with Castell Gwallter (SN 6217 8677; Scheduled Monument CD005) in the parish of Llanfuhangel Genau Glyn. Maredudd ap Grufydd ap Rhys and Rhys ap Gruffydd were rulers in Deheubarth. Nothing further is known of this event.

RCAHMW (Battlefields Inventory), Jan 2017

Bibliography
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).