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 Hayreddin Barbarossa, who offered an honourable 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.

Even the Venetian refused the promised naval link to Castelnuovo with the supplies and reinforcements; after a general troop parliament, the commanding officer Francisco de Sarmiento asked his captains what his answer should be to Barbarossa's offer to surrender.

The courage displayed by the Old Tercio of Naples during this last stand, however, was praised and admired throughout Europe and was the subject of numerous poems and songs.

[12] The Ottoman admiral was then required to return to Constantinople, where he was appointed commander of a great fleet to conduct a campaign against the Republic of Venice's possessions in the Aegean and Ionian Seas.

Barbarossa captured the islands of Syros, Aegina, Ios, Paros, Tinos, Karpathos, Kasos, Naxos, and besieged Corfu.

[12] The Republic of Venice, frightened by the loss of their possessions and the ruin of their trade, conducted a vigorous campaign for the creation of a "Holy League" to recover the lost territories and expel the Ottomans from the sea.

[12] In February 1538, Pope Paul III succeeded in creating a league which united the Papacy itself, the Republic of Venice, the Empire of Charles V, the Archduchy of Austria, and the Knights of Malta.

[20] Sultan Suleiman the Magnificent ordered Barbarossa to reorganize and rearm his fleet during the winter months to have it ready for the battle in the spring of 1539.

[20] According to the orders received, Barbarossa's army, numbering about 200 ships with 20,000 fighting men aboard, would blockade Castelnuevo by sea while the forces of the Ottoman governor of Bosnia, a Persian named Ulamen, would besiege the fortress by land in command of 30,000 soldiers.

[20] Sarmiento, meanwhile, used the peaceful months prior to the siege to improve the defenses of the town, repairing walls and bastions and building new fortifications.

[21] Andrea Doria, who was in Otranto with 47 Imperial and 4 Maltese galleys, received news of Castelnuovo's situation, but given the inferiority of his fleet he sent a message to Sarmiento recommending him to surrender.

[24] The Ottoman pioneers spent five days digging trenches and building ramparts for 44 heavy siege guns carried aboard Barbarossa's fleet or by Ulamen's troops, and even smoothed the fields around Castelnuovo to facilitate maneuvers.

[24] Another sortie by a Spanish force of 800 men surprised several units of Janissaries who were attempting to storm the walls of Castlenuovo, killing most of them and leaving the field strewn with corpses.

Two squad corporals of Captain Vizcaino's company, Juan Alcaraz and Francisco de Tapia, managed to return to Naples and write their version of events many years later.

[17] Encouraged by the successful defense, several Spanish soldiers decided to conduct a surprise raid on the Ottoman camp with the approval of Sarmiento.

[26] By nightfall the remnants of the Spanish garrison retreated to the walls of the town with their wounded, leaving the ruined castle in Barbarossa's hands.

These were immediately brought to Barbarossa and encouraged the admiral to continue with the assaults, reporting that the Spanish had suffered heavy casualties, lacked gunpowder and shot, and were mostly injured and exhausted.

[27] Despite the overwhelming numerical superiority of the Ottoman troops, the Spanish defense was successful, as no more than a tower of the wall fell to the besiegers that day.

[need quotation to verify] [29] After that he joined Machín de Munguía, Juan Vizcaíno, and Sancho Frias to lead the last stand.

[9] Half of the prisoners and all the clerics were also slaughtered to satisfy the Ottoman soldiers, who were angry at the great losses which they had suffered in capturing the city.

[9] Despite the failure of Sarmiento to retain the fortress, Castelnuovo's defense was sung by numerous contemporaneous poets and praised all over Christian Europe.

[9] The Spanish soldiers who participated in the unequal engagement were compared with mythological and classical history heroes, being considered immortal due to the magnitude of their feat.

Charles V began negotiations with Barbarossa to attract him to the imperial ranks but in vain, and turned all his efforts in a great expedition against Algiers to destroy the Ottoman sea power.

[33] This expedition, known as the Journey of Algiers, ended in a disaster as a storm scattered the fleet and the army had to be reembarked having suffered heavy losses.