In November 1235,
Blanche of Castile's nephew, King
Ferdinand III of Castile, lost his wife,
Elisabeth of Hohenstaufen, and Blanche's sister
Berengaria of Castile, Ferdinand's mother, was concerned that her widowed son might involve himself in liaisons that were unsuited to his dignity as king. Berengaria determined to find Ferdinand another wife, and her sister Blanche suggested Joan of Dammartin, whose marriage to the king of Castile would keep her inheritance from falling into hostile hands. In October 1237, at the age of about seventeen, Joan and Ferdinand were married in
Burgos. Since Ferdinand already had seven sons from his first marriage to
Elisabeth of Hohenstaufen, there was little chance of Ponthieu being absorbed by Castile. They had four sons and one daughter: •
Ferdinand II, Count of Aumale (1239 – ca 1265), who married Laure de Montfort, Lady of Espernon, daughter of
Amaury VI of Montfort, sometime after 1256 and had issue. •
Eleanor (1241–1290), Countess of Ponthieu, who married king
Edward I of England and had issue. • Louis (1243 – ca 1275), who married Juana de Manzanedo, Lady of Gaton, and had issue. • Simon (1244), died young and buried in a monastery in
Toledo. • John (1246), died young and buried at the cathedral in
Córdoba. She accompanied Ferdinand to Andalucia and lived with him in the army camp as he besieged
Seville in 1248. Upon her mother's death in 1251, Joan succeeded as Countess of Ponthieu and Montreuil, which she held in her own right. After Ferdinand III died in 1252, Joan did not enjoy a cordial relationship with his heir, her stepson
Alfonso X of Castile, with whom she quarreled over the lands and income she should have received as dowager queen of Castile. Sometime in 1253, she became the ally and supporter of another of her stepsons, Henry of Castile, who also felt Alfonso had not allowed him all the wealth their father had meant him to have. Joan unwisely attended secret meetings with Henry and his supporters, and it was rumored that she and Henry were lovers. This further strained her relations with Alfonso and in 1254, shortly before her daughter Eleanor was to marry Edward of England, Joan and her eldest son Ferdinand left Castile and returned to her native Ponthieu. ==Rule in Ponthieu and Aumale==