After the crucial 1526 Battle of Mohács and the 1527 election in Cetin, Croatia became part of the Austrian Habsburg Empire. The Ottoman Empire formally established the Eyalet of Bosnia in 1580. The Croatian lands in general were reduced to a fraction of what they encompassed, and only the westernmost parts of today's Bosanska Krajina still resisted the Ottoman rule. Nevertheless, the Ottoman armies preferred to advance towards their targets in the northwest through more easily passable terrain, such as along the river Danube, for example Vienna was first besieged in 1529 after the army had gone through Osijek, Mohács and Buda. The natural obstacles in and around the region, especially at the time, included the rivers Sava, Vrbas, Una and Sana, as well as the mountains such as Plješevica, Šator, Klekovača, Raduša, Grmeč, Kozara and Vlašić.