It is unclear whether Posculo participated in the defense of Constantinople, but the detail he is able to provide in the "Constantinopolis" about the positions of the defenders suggests he may have been among those stationed along the walls. What is more certain is that after Constantinople fell, Posculo was taken prisoner, sold as a slave, and held in Pera, thus compelling him to secure ransom. After a year his ransom had been paid, but before he was even able to leave the Aegean Sea, Posculo was captured by pirates and nearly sold into slavery a second time on the island of Rhodes. Through the assistance of Hospitaller Knights, Posculo was able to escape and eventually return to Italy in 1455.