The decade 1267-77 saw the centre of gravity in Anglo-Welsh affairs shifting to the south-east: guarding his rear by a thorough exploitation of the fiscal and military resources of the interior, Llywelyn built the castle of Dolforwyn and founded the borough of Abermule, launching attacks on his neighbours further south, and meeting with some success in his dealings with the Bohun family, but failing to unseat the Clares (qq.v.) in their new castle at Caerphilly. The evident ruthlessness of his domestic policy at this time — as witness his attitude to the northern bishops — brought to a head social and legal changes of radical import, and earned him the hostility of many subjects. The defection of Dafydd and of Gruffydd ap Gwenwynwyn (1274) was a serious blow, anticipating the wholesale desertions of 1277.

Llywelyn's dissatisfaction over mounting difficulties along the border accounts for his repeated refusals from 1273-7 to observe treaty obligations as vassal of the new king — Edward I. Though it is unknown how serious were his grievances, he has been severely criticised for his intransigence and lack of caution, to say the least, in thus challenging the revived power of the English monarchy, a policy which ended with his defeat in the war of 1277 and the collapse of his life's work. The subsequent peace of Aberconway left him with only Gwynedd west of Conway, though he was still accorded the now hollow title of Prince of Wales, with which was associated the overlordship of five small baronies on the outskirts of Snowdonia.

At Worcester, on 13 Oct. 1278, in the presence of the king, he was m. to Eleanor de Montfort, daughter of earl Simon, an alliance contracted as far back as 1265, at Pipton, when Simon and Llywelyn had formally joined forces against the Crown. The wedding had actually been postponed since 1275 owing to the bride's forcible detention, at Windsor, by Edward I.

Five fateful years followed during which Llywelyn acted with the utmost propriety towards the king, exercising, in most painful circumstances, untold patience and a high degree of diplomatic finesse. Technical legal issues bound up with the exact interpretation of the recent treaty lay at the root of the trouble in which Gruffydd ap Gwenwynwyn also had a part, although causes of friction arose outside the direct control of either party. From Llywelyn's standpoint the king was unquestionably contravening treaty terms in applying English instead of Welsh law to the matters in dispute, though his motives in so doing have been subject to varying judgements. Even then Llewelyn's hands were forced; it was Dafydd who provided the casus belli by his attack on Hawarden on Palm Sunday, 1282, thus precipitating general hostilities.

While campaigning in the Builth region Llywelyn was killed on 11 Dec. 1282, an event which virtually brought Welsh political independence to an end. He did not die in open battle, but on the banks of the Irfon, at the hands of one who was unaware at the time of his victim's identity. His head was later sent for public display in London, the body being interred by the Cistercians of Cwm Hir — one of the houses of an Order to which the prince had been much attached and in which he found some of his most loyal supporters. Eleanor had died in childbirth some little time before, and her daughter, Gwenllian, Llywelyn's sole legitimate descendant, ended her days as a nun of Sempringham.