In August of 1526, Sultan Suleiman II (also known colloquially as Suleiman the Lawgiver or Suleiman the Magnificent) smashed the forces of King Lajos II of Hungary at the Battle of Mohacs. Following the conquest and following subjugation of Hungary, Suleiman turned his attention to Austria, where Archduke Ferdinand eyed Suleiman's advances, coming to quickly realize their importance to the survival of his own kingdom, not to mention Christian Europe (although to which his sympathies lay is an unsettled question).
Three years following his conquest of Hungary, in the spring of 1529, Suleiman's armies began a general mobilization in Ottoman Bulgaria, mustering a host of around (or possibly surpassing) 325,000 men, 90,000 camels, and 500 artillery. Included among them was a force of at least 20,000 of the elite janissary corps, and a small force of Christian Hungarians fighting for their new Turkish rulers. With Suleiman acting as commander-in-chief and his grand vizier, a Greek slave known only as "Ibrahim" acting as seraskier (a commander equivalent to a Western field marshal), the army set out soon after for Vienna.
The spring rains characteristic to that part of central Europe were particularly fierce that year, making many of the major roads in the area - including those leading to Vienna - a soggy, barely-passable mess. Thousands of camels were lost when they broke their legs and had to be slaughtered, and about two hundred of the heaviest field guns were turned back due to the roads. Acting against the advice of seraskier Ibrahim, however, Suleiman pressed on, saying, "It is beneath my dignity to allow the weather to interfere with my plans." Therefore the force pressed on, intending to rely on the disciplined ranks of Balkan miners to subvert the walls and setting the stage for the macabre combat ahead.
The populace of the city reacted with terror when news reached them of the advancing Ottoman force. Stories of their inexorable approach, especially the tales of the janissaries' murderous brutality (a notable example being the massacre of the surrendering German garrison and, subsequently, the populace of the town of Pest) infused the city with first a sense of fear and then a resigned will to fight to the death that would serve it well during the siege to come. Ferdinand I, however, had none of this will - he fled to the relative safety of Hapsburg Bohemia following the rejection of pleas to his brother, Charles V, Holy Roman Emperor, for help. His designated commander, a Duke Frederick, then gave operational command of the defence to a 70 year-old German mercenary named Nicholas, Graf von Salm.
He arrived with 1,000 German Landsknechte, formidable mercenary pikemen, and another 700 Spanish musketmen. Taking charge of the garrison of 23,000 infantry, 2,000 cavalry, and 75 cannon, he moved desperately to reinforce the city's 300 year-old walls, pierced by four gates and surrounding St. Stephen's Cathedral, which he would make his headquarters. He ordered the digging of fireproof magazines and erected earthen bastions for defenders to mount in case the wall (reinforced by paving stones) should be breached. Flammable shingles were torn from the roofs, and the four gateways were heavily reinforced.
In a move to preserve what food the city had stored in the event of siege, Salm ordered about 4,000 women, children, and elderly men out of the city via an escorted column. However, by this time lower Austria had been inundated by the Ottomans' advance parties, and most of the group was slaughtered 'en masse' at Traismauer, with particular cruelty being shown to some who were impaled on stakes (although some young women were left alive to be sold as slaves). Resistance within the city - which now saw itself as the final bulwark for Christianity - was uplifted and entrenched, even as Austria burned around the helpless defenders. By late September - two months tardy - the Ottoman host had arrived.
On 1 October, however, a miner of Christian parentage that escaped into the city reported that the real purpose of the bombardment was to mask tunnelling efforts beneath the city. The Carinthian Gate, one of the city's four entrances, was the apparent target of this new assault, with the intent being to blow up the towers and then attack with assault troops. Salm, an expert in tunnelling, quickly took ingenious steps against the efforts, including placing buckets of water and dried peas near the cellar walls of homes adjacent to the gate. When they shook, an alarm was sounded and counter-miners commenced digging "like moles". What they discovered were six different tunnels, quickly moving through the Earth and towards the helpless bastions.
The Austrians dug until they struck the enemy tunnels, some of which were deserted, with powder kegs ready to be lit (these were carried off as booty) and some which were still occupied by miners and immediately became the site of bizarre combat. Guns were unusable due to the proximity of the kegs, so the men fought with whatever tools or body parts they could muster, each blow finding a target and countless fighting dying like, in the words of one witness, "devils from the nether pit of hell", returning above approaching insanity and covered with blood. As the battle continued, new weapons were devised for the underground war, including Turkish cavalry maces and Austrian sharpened spades as the fighting below ground grew even more fierce. At one point, a powerkeg prematurely exploded, killing dozens on both sides. The total death toll from this mini-war is still unclear, but it was here originated the term that would come to define the battle, the Siege of the Moles.
The majority of the mines were discovered before any damage could be done, but constant digging exhausted Viennese capabilities and on 5 October two mines exploded beneath the Salt Gate, leaving room enough for a company of soldiers to break through. Janissaries immediately stormed the breach but were met by twelve-foot pikes and halberds, repulsing them with heavy losses. Within hours the breaches were refilled.
The night after, the Austrians replied with a new form of deadly assault. Dozens or possibly hundreds of fanatics wearing cloaks of black and armed with homemade bombs - quite possibly one of the first appearances of the Molotov cocktail - exited the city in silence and strode into the Ottoman camps, tossing their bombs into tents before making their escape. As many as 2,000 Turks died unawares, sleeping.
The fighting continued unabated. Some days later a mine finally brought down the two towers of the Carinthian Gate, bringing them down and opening it to assault, but the advance was held back by Spanish harquebusiers, German pikemen and Bohemian two-handed swordsmen with a heap of 1200 janissary dead left behind at the end.
On 14 October, the attack began, with seraskier Ibrahim leading the charge personally. The drives were aimed at the ruined Carinthian Gate and the bastion nicknamed The Berg. The assault was led by bashi-bazouks, militia intended to tire the Austrians with sheer crush of humanity, to be followed by janissaries, who for the first time were offered a bounty of silver where heretofore only fervor had sufficed. Suleiman ordered that regardless of losses the attack be launched three times - he would either win or suffer dearly in the attempt.
One mine failed to blow, but the other succeeded and with screaming battle cries the Turks broke through the breaches only to be faced again with palisades and long pikes. The baski-bazouks charged twice and fell back twice as they were cut down again and again by pike and musket. The janissaries too struck with murderous fervor and were too beaten back, with piles of bodies collecting where they tried and failed. Hand bombs rained upon them as grapeshot from cannons on the Berg cut through their ranks. Salm had descended from St. Stephen's to assume personal command of the battle, almost immediately being hit, a wound from which he would soon after die.
Finally, without being ordered, the janissaries fell back despite Ibrahim's efforts to whip them into another charge. They immediately swarmed back to the camp and struck their tents, unpursued. The siege was over.
Overall the invasion and subsequent siege required a ghastly price from both sides, with tens of thousands of soldiers and civilians dead and thousands more sold into slavery. Practically, it marked the end of the Ottoman advance into Europe and the beginning of their decline from dominant power in the Renaissance world. While they would remain strong for many years after - even mounting another siege, resulting in the Battle of Vienna a hundred years later, they would never again reach their prior heights of power.