Gruffudd ap Llywelyn, Prince of Gwynedd. [Burke's Peerage]
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The following post to SGM, 17 Feb 2003, by Becky Thill is mostly about Gruffudd's brother Dafydd, who was a disastrous Prince of Cymru for 6 years, but it had the following excerpt on Gruffudd:
But Davydd was not the only son of Llywelyn. There had been another, Gruffydd, a handsome giant of a prince, born of a Cymric Mother. Llywelyn had weighed his two sons in his mind as to which would be the better ruler for the land. Davydd's mother was sister of the King of England - surely the King of England would be less the enemy of his nephew than of Gruffydd. So thought Llywelyn, and accordingly he made Davydd his heir; the one tragic mistake which undid all that he had accomplished in his own strenuous life. Gruffydd, with his turbulent valour, could not have brought his country lower than his brother's smoothness brought it. He might have saved it.
Davydd had handed over his brother Gruffydd to the King of England as a prisoner. The king sent him to the Tower of London. One night Gruffydd made a rope of his clothes and began to let himself down out of his window to escape. The rope broke, and Gruffydd was killed, his neck being broken by the fall. But he left four sons, Owen, Llywelyn, Davydd and Rhodri.
Owen and Llywelyn seized the crown of Gwynedd between them when their uncle Davydd died. Now King Henry claimed all the land as his own, for, amongst the rest of Davydd's doings, he had agreed that, if he died childless, Henry should be his heir and take the country. It would be hard to find words severe enough to describe such a transaction as that. [Flame-Bearers of Welsh History, "The Sons of Cunedda" by Owen Rhoscomyl, School Edition, 1905 The Welsh Ed. Pub. Co. Merthry Rydfil, part one, p. 177 XLIX, THE RISE OF LLYWELYN III]