Third and youngest daughter of Don Pedro the First, surnamed The Cruel, and Maria Padilla, whose marriage is usually considered a fiction by modern writers, though Pedro himself solemnly affirmed it, and their daughters were treated as Princesses through life. Isabel was born at Morales or Tordesillas, in 1355. In 1365, when Don Pedro fled before his rebel brother, he was accompanied by his third wife, Juana, and his three daughters, Beatriz, Constanga, and Isabel. They fled from Sevilla to Bayonne, and did not return to Sevilla till 1368, after the victory of Navareta. After the loss of the battle of Montiel and the murder of their father, in 1369, the Princesses were hastily taken again by their guardians to Bayonne. Constancy was married to John of Gaunt, Duke of Lancaster, at Rochefort, near Bordeaux, about November, 1369. Isabel remained with her sister, and accompanied her to England in 1371. In 1372--between January 1st and April 30th--she married Edmund Earl of Cambridge, the brother of her sister's husband. It was at Hertford, March 1st, 1372, that John of Gaunt and Constanca assumed the titles of King and Queen of Leon and Castilla; and as sixteen months had then elapsed since their own marriage, the probability seems to be that this date marks the marriage of Isabel, and the consent of her bridegroom to the exclusive assumption of queenship by the elder sister. (The other and really eldest sister, Beatriz, had become a nun.) Isabel is alluded to as Edmund's wife on April 30th, 1372. In May, 1381, she accompanied her husband to Portugal, on an expedition undertaken with the object of securing the recognition of herself and her sister as the true heirs of Castilla. The expedition failed; and Isabel returned to England with her husband in October, 1382.