What year did Transylvania become a territory of the Habsburg ' Austrian empire ?
Rákóczi 's defeat in Poland , combined with the subsequent invasions of Transylvania by the Turks and their Crimean Tatar allies in 1660 , the ensuing loss of territory ( most importantly , the loss of the most important Transylvanian stronghold , Oradea ) and diminishing manpower led to the complete subordination of Transylvania , which became a powerless vassal of the Ottoman Empire .