William IV died soon after inheriting the castle, but William V, who came of age in 1200, was increasingly involved in wider baronial politics, including serving in Ireland with King John, and taking part in the First Barons' War. He sided with the rebels against John during the civil war and was captured at Rochester Castle, leading to his estates, including Eynsford, being forfeited to the Crown. He recovered them after the end of the war and later became the constable of Hertford Castle. His daughter's son, William VI, inherited the castle in 1231; around this time, the hall of the castle burnt down and was rebuilt, complete with glazed windows. Both William VI and his son William VII died young, ending the family line in 1261.