The years that followed the death of Llywelyn ab Iorwerth saw a major reversal in the fortunes of Gwynedd, culminating in the Treaty of Woodstock, concluded with King Henry III in 1247, a year after the death of Llywelyn's son Dafydd. Under the terms of the treaty, Gwynedd lost all its lands to the east of the River Conwy.

In 1255, Llywelyn ap Gruffydd defeated his brothers Owain (d.1282) and Dafydd (d.1283), in battle at Bryn Derwin. Owain was imprisoned, and Llywelyn set about reasserting the authority of Gwynedd and thereafter extending it into a supremacy over much of the rest of Wales. In 1267 his position as overlord was recognized by Henry III in the Treaty of Montgomery when the English king accepted Llywelyn's homage as prince of Wales.

Ten years after his recognition as the prince of Wales by Henry III, Llywelyn was to suffer a humiliating defeat at the hands of the new King Edward I. From the outset, Llywelyn seemed almost to go out of his way to court Edward's anger. In particular, he refused to yield the homage and money payments owing to the king under the terms of the Treaty of Montgomery. He tempted fate further by arranging to marry Eleanor, daughter of rebel baron Simon de Montfort, an act destined to strain Edward's patience to the limit.

Llywelyn began his own castle campaign by strengthening his grandfather's castles at Criccieth, Ewloe, and Dolwyddelan. Moreover, in 1273 he started to build a new castle at Dolforwyn, high above the Severn valley, posing a challenge to the royal frontier post at Montgomery. The prince's refusal to abandon this project was just one incident in an eventual catalogue of disagreements with the new king.

Enough was enough, and in 1276-77 Edward had decided to settle accounts with the recalcitrant Welsh prince. Edward himself took the field at Chester in July 1277, and by August he had some 15,600 troops in his pay. Against these odds, Llywelyn had no choice but to sue for peace. The ensuing Treaty of Aberconwy represented a comprehensive humiliation for the prince of Wales.

Stripped of his overlordship he had won ten years earlier, Gwynedd was again reduced to its traditional heartland to the west of the River Conwy.

On 21 March 1282, Llywelyn's brother, Dafydd, attacked Hawarden Castle and sparked off the war of 1282-83. Llywelyn was faced with an almost impossible dilemma. Torn between his fealty to the king and his loyalty to his brother Dafydd and his people, Llywelyn was to side with his brother and led the Welsh resistance to the inevitable invasion by Edward I. By the end of the year Llywelyn was dead, having been killed on 11 December in a brief engagement with English forces at Irfon Bridge near Builth Wells.