[2] He is known for his role in the capture and ultimately turning over of Nymphaeum to the Bosporans, for which he was punished.
Nymphaeum fell under Athenian rule when Gylon captured the city and established a garrison there,[3] along the strategically important grain route.
[5] Gylon is said to have betrayed Nymphaeum to "the enemy"[1] when in 405 BC, he handed it to the Bosporan King Satyros[2][6] in exchange for needed grain.
One theory is that it was no longer feasible for the Athenians to maintain the outpost.
He was first sentenced to death, but the punishment was later changed to a fine that Demosthenes paid off.