He relied on his kinsmen, especially the Tudors of Anglesey who captured Conwy castle (April 1401).

He was proclaimed prince of Wales by friends and relatives at Glyndyfrdwy (Merioneth) on 16 September 1400, and attacked Grey's estates and towns close to the English border.

Glyndŵr took the lead partly because of personal grievances against Lord Grey of Ruthin and Henry IV. French meddling and encouragement from Scotland and Irish lords fed the resentment, and risings before 1400 (especially by Owain Lawgoch in the 1370s) were a foretaste of widespread unrest. By 1400 conditions in Wales were ripe for rebellion: there was racial tension between Welsh and English fanned by poets and propagandists economic and social dislocation after the Black Death was made worse by the crown and English nobles exploiting their Welsh resources more harshly and the Welsh church was dominated by an English hierarchy at the expense of Welsh priests. He married Margaret, daughter of Sir Edward Hanmer, a distinguished judge. Owain was a well-to-do gentleman, trained at the Inns of Court, serving in Richard II's armies against the Scots (1384–5), and becoming a retainer (by 1387) of the lord of Chirk, Richard Fitzalan, earl of Arundel. His rising (1400–10) was a serious and costly threat to the usurper Henry IV. A wealthy landowner in north-east Wales, his father was descended from princes of Powys and his mother from princes of Deheubarth. Glyndŵr, Owain ( Owain Glendower) ( c.1359– c.1415), self-styled prince of Wales.

Owain Glyndwr by Terry Breverton