Zápolya would eventually seek aid from, and become a vassal of the Ottoman Empire, after Ferdinand began to take control of western Hungary, including the city of Buda.

Ferdinand won recognition only in western Hungary; a noble called John Zápolya, from a power-base in Transylvania, challenged him for the crown and was recognised as king by Suleiman in return for accepting vassal status within the Ottoman Empire.

[12] In the spring of 1529, Suleiman mustered a large army in Ottoman Bulgaria, with the aim of securing control over all of Hungary at his new borders by Ferdinand I and the Holy Roman Empire.

[14] Suleiman acted as the commander-in-chief (as well as personally leading his force), and in April he appointed his Grand Vizier (the highest Ottoman minister), a Greek former slave called Ibrahim Pasha, as Serasker, a commander with powers to give orders in the sultan's name.

[16] The spring rains that are characteristic of south-eastern Europe and the Balkans were particularly heavy that year, causing flooding in Bulgaria and rendering parts of the route used by the army barely passable.

Many large-calibre cannons and artillery pieces became hopelessly mired or bogged down, leaving Suleiman no choice but to abandon them,[17] while camels brought from the empire's Eastern provinces, not used to the difficult conditions, were lost in large numbers.

[16] As the Ottomans advanced towards Vienna, the city's population organised an ad-hoc resistance formed from local farmers, peasants, and civilians determined to repel the inevitable attack.

The defenders were supported by a variety of European mercenaries, namely German Landsknecht pikemen and professional Spanish harquebusiers, sent by Charles V, who was Holy Roman Emperor and King of Spain.

This elite infantry excelled in the defense of the northern area and with discretion fire prevented the Ottomans from settling in the Danube meadows, near the ramparts, where they could have breached with enough space to work.

[citation needed] The Hofmeister of Austria, Wilhelm von Roggendorf, assumed charge of the defensive garrison, with operational command entrusted to a seventy-year-old German mercenary named Nicholas, Count of Salm, who had distinguished himself at the Battle of Pavia in 1525.

[16] Salm arrived in Vienna as head of the mercenary relief force and set about fortifying the three-hundred-year-old walls surrounding St. Stephen's Cathedral, near which he established his headquarters.

[16] The Ottoman army that arrived in late September had been depleted during the long advance into Austrian territory, leaving Suleiman short of camels and heavy artillery.

[31] The two Viennese campaigns in essence marked the extreme limit of Ottoman logistical capability to field large armies deep in central Europe at the time.

The outcome of the campaign was presented as a success by the Ottomans, who used the opportunity to show off their imperial grandeur by staging elaborate ceremonies for the circumcision of princes Mustafa, Mehmed, and Selim.

[33] Ferdinand I erected a funeral monument for the German commander Nicholas, Count of Salm, head of the mercenary relief force dispatched to Vienna, as a token of appreciation of his efforts.