April 11, 1240 - Llewelyn Fawr (the Great), Prince of Gwynedd, died at the Aberconwy Abbey and was buried there. Llewelyn's dream was a strong, united Wales. He married Joanna, daughter of the English King John.
Sharon Kay Penman's excellent novel Here Be Dragons is about Llewelyn's life. If you haven't (what's wrong with you?) you need to!