The Siege of Vienna of 1529 (to be distinguished from the Battle of Vienna in 1683) represented the westernmost advance of the Ottoman Empire, and of all the clashes between the armies of Christendom and those of the 'Islam. He can be credited with being the one who finally blocked the Turkish forces (although they later conquered the parts of Hungary held by Austria).
The Ottomans
Sultan Suleiman I had launched an expansion of his empire:1521 - takes Belgrade 1522 - seizes Rhodes, held by the Knights of St. John of Jerusalem and Rhodes 1526 - battle of Mohács against the Austrians and Magyars, capture of Buda and most of Hungary 1527:capture of Bosnia, Croatia, Slavonia and Dalmatia (from the Venetians)
Seeing Austria as a powerful enemy, he intended to launch an attack directly at its center, the capital. There the Archduke Ferdinand watched the advance including the rest of Western Europe (which was not all favorable to him, in particular Francis I).
In the spring of 1529 Suleyman mobilized a large army of at least 100,000 men and 500 pieces of artillery. There were at least 20,000 Janissaries and a few Hungarian knights fighting for their new master. Suleiman was the commander-in-chief and grand vizier while a Greek slave known only as "Ibrahim" acting as a seraskier had responsibility for coordinating the troops.
The spring rains were particularly heavy this year making the roads muddy and difficult for hundreds of camels. Two hundred guns had to turn back. The Turks relied on Balkan miners to tear down the fortress walls.
The Austrians
The population of the city reacted with terror when the news reached them of the atrocities committed by the Ottoman forces but turned into a fierce will to resist. Ferdinand left for the relative safety of Bohemia following his brother Charles V's refusal to help him. He appointed Duke Frederick as commander who gave control of the defense to a 70-year-old German mercenary named Nicholas, Count von Salm.
He came with 1000 pikemen and 700 Spanish musketeers. Taking charge of the garrison of 23,000 soldiers, 2,000 cavalry and 75 guns, he hastily reinforced the walls of more than 300 years. He ordered the digging of fireproof magazines and barricades in case the walls fall.
In order to spare the food reserves he ordered 4,000 women, children and old men out of the city in an escorted column. However, Lower Austria was flooded by Ottoman scouts and most of this group was massacred and then subjected to the torture of the pale. Children and young women were enslaved.
The Turkish army arrived in September. A party was sick and among the able-bodied a third were light cavalry, therefore not very useful for a siege. The emissaries were received by von Salm who refused to surrender.
The next day 300 guns opened fire simultaneously, the gunners having made great efforts to keep the powder dry, but the result was negligible. Flaming arrows had little effect.
The response was a surprise raid by a hundred horsemen under Eck von Reischach which killed two teams of gunners before returning to the safety of the walls. The bombardment continued with as little result as ever and there was no indication of an assault.
Moles
On October 1 a Christian miner who had managed to escape into the city reported that the real reason for the cannonade was to drown out the sounds of tunneling into the city. The Carinthian Gate, one of the four gates, was the apparent target. Salm, a tunnel expert, quickly took ingenious measures against these efforts, including placing buckets of water and dried peas near the cellars near the door. When they moved an alarm was given and counter-miners began digging, uncovering six tunnels. Some already had powder barrels and others still had the miners for the Ottomans. As the use of pistols was impossible it was a fight with knives. Those who came back were covered in blood. One moment a barrel exploded killing dozens of men on each side. We do not know the losses, but it will make this seat called "the seat of the moles".
The majority of the mines were discovered before any damage could be done but the continual digging exhausted the defenders and on October 5 two exploded near the salt gate, leaving enough room for a company of soldiers to enter. The Janissaries rushed in but they were welcomed by the pikemen and had to retreat after heavy losses.
The following night the Austrians responded with a new form of assault. Dozens and perhaps hundreds of fanatics wearing black cloaks and pipe bombs came out silently and crept up to the Ottoman camp threw them at the tents before fleeing. 2,000 Turks perished in their sleep.
The fight continued unabated. Another explosion at the Corinthian Gate brought an attack by the Janissaries which was repelled by arquebusiers and Bohemian warriors with two-handed swords.
However on October 11 the rain continued and other camels fell ill. In addition, the Viennese began to mount cannons on the roofs, including the "royal" ones, which therefore had more range than those of the Turks. Food for the besiegers was scarce. Soliman held a council of war and a final assault was decided.
On October 14 the attack began with seraskier Ibrahim leading the charge personally towards the Carinthian gate and the bastion called the castle (Berg) with the Bashi-bouzouk, militia, followed by the janissaries who for the first time had a promise of booty while ordinarily fervor should suffice. Suleiman ordered the attack three times regardless of casualties. Salm himself came to participate but was immediately seriously injured and died shortly afterwards.
After this new setback, Sultan Suleiman consulted with his commander-in-chief Ibrahim Pasha. At the end of this council of war, he gave the order to beat a retreat towards Constantinople. The sultan may have understood that his soldiers needed rest, that they were exhausted and that the morale of the troops was at an all-time low. Perhaps he also feared an early arrival of winter. He may also have heard of the advance towards Vienna of the relief army under the command of Frederick, Count Palatine of the Rhine. But in reality, this army was far too weak to challenge the Turks on a battlefield.
That same night, the Turks began to evacuate their outposts. Their retreat was punctuated with atrocities:they murdered many prisoners - there were more than 2000 victims, according to the statements - taking only the young and healthy people, then condemned to a life of slavery, and massacring the old and the destitute, whose cries of distress echoed above the city walls. However, gaiety reigned the next morning, the rumor of the Turkish retreat having become a certainty. A solemn mass was celebrated in St. Stephen's Cathedral (Stephansdom), the bells of the churches of the city were ringing at full speed. Vienna was saved!