Isabella of Castile (c. 1355 – 23 December 1392) was the youngest of the three daughters of King Peter of Castile by his favourite mistress, María de Padilla (d.1361).[2] On 21 September 1371 Edward III's fourth son, John of Gaunt, 1st Duke of Lancaster, married Isabella's elder sister, Constanza (d.1394), who after the death of her father in 1369 claimed the throne of Castile. Isabella accompanied her sister to England, and on 11 July 1372, at about the age of 17, married John of Gaunt's youngest brother, Edmund of Langley, 1st Duke of York, fifth son of King Edward III and Philippa of Hainault, at Wallingford, Oxfordshire, as part of a dynastic alliance in furtherance of Gaunt's claim to the crown of Castile.[3] According to Pugh, Isabella and Edmund of Langley were 'an ill-matched pair'.[4]