Unearthed in an archaeological dig in the contemporary Israeli coastal city of Ashdod, the heart of the well-preserved fortifications is a mud-brick wall more than 12 feet thick and 15 feet high, researchers from Tel Aviv University reported.
Stretching for hundreds of feet, the fortifications would have formed a daunting crescent-shaped defense for an inland area covering more than 17 acres, they said.
"The fortifications appear to protect an artificial harbor," archaeologist Alexander Fantalkin said. "If so, this would be a discovery of international significance, the first known harbor of this kind in our corner of the Levant."
When the fortifications were built in the 8th century B.C., the Assyrians ruled the southeastern part of the Mediterranean basin, including parts of Africa and the Middle East.
The fortifications may have been built during a local rebellion against Sargon II, the king of the Assyrian Empire, which was brutally put down by the Assyrians, researchers said.
"An amazing amount of time and energy was invested in building the wall and glacis [embankments]," Fantalkin said.