[7] Suleiman secured his possession in Hungary by conquering several other forts,[10] but after the Ottoman withdrawal, Habsburg Emperor Ferdinand I reoccupied some of the devastated territory.An assault of Buda was driven off by John Zápolya, the vassal King of Hungary, but Ferdinand was successful elsewhere, capturing Gran (Esztergom) and other forts along the Danube river, a vital strategic frontier.[2] During the early period of the Little War in Hungary, Suleiman, as a response to Ferdinand's counter-attack in 1530, and as a part of his fifth imperial campaign[16] (Ottoman Turkish: سفر همايون, Sefer-i humāyūn)[Note 3] in 1532, led a massive army of over 120,000 troops to besiege Vienna again.[19] After Suleiman crossed the river Drava at Osijek, instead of taking the usual route for Vienna, he turned westwards into Ferdinand's Hungarian territory.[2] According to historian Andrew Wheatcroft, on the route for Vienna, the Ottoman army had briefly invested and captured seventeen fortified towns or castles.[2] However, in order to make decisive gains, the Ottomans had to take the city quickly, as a large Imperial army, raised in Germany, reinforced by Spanish troops and led by the Emperor Charles V himself was approaching in support of Ferdinand.[2] For more than twenty-five days, without any artillery, Captain Nikola Jurišić and his garrison of 800 Croat soldiers held out against nineteen full-scale assaults and an incessant bombardment by the Ottomans.[7] According to historian Paolo Giovio, Charles arrived with an imperial army at Vienna on 23 September, too late to fight the Ottomans because Suleiman had already withdrawn.