In 1401, he and his brother Rhys ap Tudur took Conwy Castle after infiltrating it, in support of their cousin Owain Glyndŵr.
[2] Along with his brother Rhys, Gwilym was one of the leaders of a Carmarthenshire contingent of soldiers which had been raised in 1386 as a precaution against a French invasion via Ireland.
[1] While Owain's rebellion in North East Wales faltered, Rhys and his family rose up against the king on Anglesey in September of that year.
Henry IV personally took an army to put down the revolt, and harried the island, burning the Franciscan Llanfaes Friary near Bangor, Gwynedd, where the Tudur family were buried.
[5][6] When Henry issued a general pardon for those of North Wales in March 1401, he purposely excluded Gwilym, his brother Rhys and Owain Glyndŵr.