However, the Hungarian-Croatian Kingdom was not enough well prepared and organized and the Ottoman Empire expanded further in the 16th century to include most of Slavonia, western Bosnia and Lika. For the sake of stopping the Ottoman conquering and possible assault on the capital of Vienna, the large areas of Croatia and Slavonia (even Hungary and Romania) bordering the Ottoman Empire were organized as a Military Frontier which was ruled directly from Vienna military headquarters. The invasion caused migration of Croats, and the area which became deserted was subsequently settled by Serbs, Vlachs, Germans and others. The negative effects of feudalism escalated in 1573 when the peasants in northern Croatia and Slovenia rebelled against their feudal lords due to various injustices. After the fall of Bihać fort in 1592, only small areas of Croatia remained unrecovered. The remaining were referred to as the "reliquiae reliquiarum of the once great Croatian kingdom".