Forum of Theodosius (Theodosius Forumu) Istanbul
The Forum of Theodosius was probably the largest square in Constantinople and stood on the Mese, the major road that ran west from Hagia Sophia. It was originally built by Constantine I, probably on the site of a pre-existing Hellenistic agora called the Strategion, and named the Forum Tauri. 

Having passed through Theodosius's triumphal arch, the Mese continued on to Thrace and the Balkan peninsula. The triumphal arch and the ancient buildings around it (to which surviving ruins in the area possibly belong) were destroyed as a result of invasions, earthquakes (the central arch and the statue of Arcadius collapsed in 558; the rest of the arch was destroyed by the Constantinople earthquake of 740) and other natural disasters from the 5th century onwards. They were completely destroyed long before the Fall of Constantinople in 1453.
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