Take the Keys & Lock Them Up  the longest royal imprisonments   Eleanor of Brittany (39 years) Daughter of Geoffrey, the son between Richard I & John, & Constance of Brittany, Eleanor & her brother Arthur arguably had a stronger claim to the English throne than Uncle John. John placed Eleanor in captivity in August 1202, where she remained until her death at age 57 in August 1241. She was never allowed to marry, which considering the contortions the Plantagenet family tree already had, probably was a dynastic blessing.   Isabella, the She-Wolf of France (28 years, 4 months) Daughter of Philippe IV of France, wife of Edward II of England, & mistress to Marcher baron Roger Mortimer, she was forced into imprisonment for her complicity in the death of her spouse. Her son, Edward III, was the jailer who made her the Chatelaine of Castle Rising from April 1330 until her death in August 1358.   Robert of Normandy (27 years, 4 months, 14 days) This eldest son of William the Conqueror succeeded his father as Duke of Normandy, but the crown of England went to his younger brother, William II. Sibling Henry I succeeded to the English throne & invaded Normandy in 1106, taking big brother prisoner at Tinchebrai. Robert remained a captive at Cardiff Castle until his death at age 82 in February 1134.   Mary Queen of Scotland & the Isles (19 years, 7 months, 10 days) Mary was confined at Lochleven by her own lords for 11 months until her dramatic escape into England. There cousin Elizabeth I proved less than welcoming, taking Mary into custody at Carlisle Castle & keeping her in various Crown strongholds until her execution at Fotheringhay Castle on February 8, 1587.   Ieuaf ap Idwal of Gwynedd (19 years) Ieuaf was co-ruler of Gwynedd with his brother Iago. As in most Welsh royal families, internecine strife broke out between them & Iago emerged victorious, capturing Ieuaf in 969. Though Ieuaf was avenged by his son Hywel's trouncing of Iago in 980, Hywel wanted the throne of Gwynedd all to himself & kept Dad locked up until Ieuaf's death in 988.