The Propylon of Diogenes was a colossal columnar facade which closed the east end of the Urban Park. Recent research on the family of its builder, one Diogenes the Younger, son of Menandros, has established that it was built near the end of the first century AD.
The structure consisted of an eight-bayed, two-storeyed columnar scaenae frons (stage front) framed by two pyrgoi (towers), beneath which ran barrel-vaulted entrance tunnels. This façade accumulated a great display of portrait statuary representing emperors and local benefactors. A deep basin or nymphaeum was constructed in front of the façade in the six century AD from re-used material, including a remarkable series of balustrade reliefs with mythological subjects taken from an unknown second-century building.