The Commonwealth fleet blockaded Flemish ports but, to Cromwell's annoyance, the military campaign started late in the year and was subject to many delays. Marshal Turenne spent the summer of 1657 campaigning against the Spanish in Luxembourg and made no move to attack Flanders until September. Mardyck was captured on 22 September and garrisoned by Commonwealth troops. Dunkirk was besieged in May 1658. A Spanish relief force attempted to lift the siege but was defeated on 4 June at the Battle of the Dunes. The Commonwealth contingent in Turenne's army fought with distinction and impressed their French allies with a successful assault up a strongly defended sandhill 150 feet high during the battle. When Dunkirk surrendered to Turenne on 14 June, Cardinal Mazarin honoured the terms of the treaty with Cromwell and handed the port over to the Commonwealth, despite the protests of Louis XIV. The Commonwealth also honoured its obligations in respecting the rights of the Catholic populations of Mardyck and Dunkirk. A contingent of Commonwealth troops remained with Turenne's army and were instrumental in the capture of Gravelines and other Flemish towns by the French. With the privateering threat of Dunkirk out of the way, England's mercantile fleet suffered far fewer losses; not only because the Dunkirkers had lost their largest base but also because English trade had already been largely lost to the Dutch as a result of Dunkirker privateering.