Answer:
Babylon
Step-by-step explanation:
Babylon was the capital of the Babylonian Empire, now its ruins are found in present day state of Iraq. About some 4000 years ago a port located in the Euphrates River became one of the largest and most powerful cities in the pre Christian world.
Hammurabi, ruled the city, and its descendants unti they felt to Cyrus the Great, head of the Persian Empire, in the Battle of Opis.
Babylons walls were considered the strongest fortress of the time, and acces was only possible through the gates present in the Euphrates River.
The walls turned to be not an obstacle for the Persian invasion, see the map below :)