During the period 1400 -16, Prince Owain Glyndŵr was fighting a War against England to regain Welsh Independence. He had most of Wales under his control by 1404 and used Cefn Caer as a base whilst he set up his Welsh Parliament at Machynlleth and whilst he and his officials organized his Coronation ceremony and associated celebrations in that town.
It is also reputed that he and his advisors penned the famous Pennal Letter whilst staying at Cefn Caer. This important document, which sought the military assistance of the French King Charles V1, gives us today a clear indication of how Prince Owain Glyndŵr and his advisors were planning for the future of the Welsh nation and its people. The Pennal Letter was dispatched via Prince Owain's envoys, Hugh Eddouyer and Maurice Kerry on March 31st 1406; it has survived to this day and is kept at the Archives Nationales, Paris.