Gwenllian of Wales

"Gwenllian ferch Llywelyn (June 1282 – 7 June 1337) was the only child of Llywelyn ap Gruffudd, the last native Tywysog Cymru (Prince of Wales)."

http://en.wikipedia.org/wiki/Gwenllian_of_Wales

"Gwenllian was placed in the Gilbertine Priory at Sempringham, where she was held behind its high walls until her death 54 years later. In committing her to a convent, Edward's aim was not only to prevent her marrying and having children who might lay claim to the Principality but also to hide her away from the outside world; unseen is unknown. He chose the Gilbertines in Lincolnshire because of their remote location in the fenlands and because they were an order where the nuns were hidden from view behind high walls. It has been speculated that the girls were taken to Lincolnshire from Gwynedd by sea."