The Siege of Castelnuovo was an engagement during the Ottoman-Habsburg struggle for control of the Mediterranean, which took place in July 1539 in the walled town of Castelnuovo, present day Herceg Novi, Montenegro. Castelnuovo had been conquered by elements of various Spanish tercios the year before during the failed campaign of the Holy League against the Ottoman Empire in Eastern Mediterranean waters. The walled town was besieged by land and sea by a powerful Ottoman army under Admiral Hayreddin Barbarossa, who offered an honorable surrender to the defenders. These terms were rejected by the Spanish commanding officer Francisco de Sarmiento and his captains even though they knew that the Holy League's fleet, defeated at the Battle of Preveza, could not relieve them. During the siege the Barbarossa's army suffered heavy losses due to the stubborn resistance of Sarmiento's men. However, Castelnuovo eventually fell into Ottoman hands and almost all the Spanish defenders, including Sarmiento, were killed. The loss of the town ended the Christian attempt to regain control of the Eastern Mediterranean.