As this battle followed the two month Siege of Vienna, and is the start of the Great Turkish War, what was the worst territorial possession lost by the Ottomans to the advancing Europeans? What was the oldest Ottoman territorial possession lost due to this war?
I'll try to answer your question, sorry if it doesn't satisfy you.
Little is known, but most eventually settled all around Germany. There are accounts of former Ottoman prisoners taking up trades and various professions, but most of them ended up joining courts of nobles. These former prisoners are known as Beutetürken (Turkish Prey). Nobles all around Germany took or bought Ottoman prisoners to join their court as footmen for ceremonies or servants, as having a Turkish servants were seen as a status symbol.
Most of them converted because of their environment in the court that supported naturalization and integration to the German society and the patronage system, where Turks who converted had better chance of stable employment by their masters and a lot of former Muslims ended up as tax collectors, priests, and staff in the bureaucracy. Most of them married into the German middle class and nobility too. For example Fatima, a former camp follower in the Ottoman army was baptized and taken as mistress by king Augustus the Strong. Another example was Mehmed, son of governor of Morea was taken prisoner, converted, taken as a valet by George I of Hannover, eventually elevated to the ranks of nobility and was known as Ludwig Maximilian Mehmet von Königstreu.
Those who refused to convert stuck as servants or courtiers. Such is the fate of two Mehmed and Hasan, whose graves can still be seen outside the Neustädter cemetery. Mehmed and Hasan both taken as prisoner after the second siege of Vienna and presumably served as servants in the court of Sophie of the Palatinate. Both of them died in 1691, and was buried in Neustädter, interestingly according to Islamic rites (positioned to face towards Mecca). Mehmed's tombstone contains German and Ottoman Turkish inscription, but Hasan's tombstone is empty for an unknown reason. The German inscription explaind that he was captured in the battle of Parkany, taken by a captain, given to the Herzog and ended up in Sophie's court. The Ottoman inscription suggests that he was a Sipahi, probably from Temeşvar.
Bibliography/Further Reading:
"Türkische Kriegsgefangene in Deutschland nach den Türkenkriegen" by Otto Spies