The Harbour of Theodosius
The Harbour of Eleutherios (Greek: λιμήν Ἐλευθερίου), later known as the Harbour of Theodosius (Greek: λιμήν Θεοδοσίου, Latin: Portus Theodosiacus) was one of the ports of ancient Constantinople, the capital of the Byzantine Empire, located beneath the modern Yenikapi neighbourhood of Istanbul, Turkey.
The harbour was located on the south side of the peninsula where the city is built, facing towards the Sea of Marmara. The other harbours of the city were the Harbour of Julian and the small harbour of the Boukoleon Palace, likewise on the southern shore, and the harbours of Neorion and Prosphorion on the northern side. The harbour was built in the late 4th century during the reign of Theodosius I, and was the city's major point of trade in Late Antiquity. The area was later transformed for agricultural use due to the effects of erosion and silting. In Ottoman times, the area was built over.
Here is a video about the harbour: