The Walls of Babylon are a Babylonian unique building, replacing the standard city Walls. The Walls of Babylon not only significantly boost the city's defensive capabilities (providing twice as many HP as normal Walls) while costing less Production to build, but they also allow the city to deal greater combat damage when conducting any bombardments.
Originally included on Antipater's list of the Seven Ancient Wonders of the World, the Walls of Babylon encapsulated the city and protected it from harm, circling the city on all sides and even spanning the Euphrates River (which ran through the center of the city). The outer walls ran 10 km (6 miles) in length, measured 8 meters (25 feet) thick, and were said to tower 98 meters (320 feet) over the city. Smaller walls ringed the city behind this primary one, assuring an extra line of defense if the first ever fell. Some 250 towers dotted the length of the walls, providing excellent look-out points and combat stations for the skilled Babylonian archers. The walls were impenetrable by any technology available to sieges at the time, large metal gates were installed at the river's ends (preventing any underwater intruders), and eight massive gates were constructed to contain any foot traffic (the most famous being the Ishtar Gate). These walls effectively protected the city for almost one hundred years.
It wasn’t until 539 BC that a way around the wall's defense was devised; Cyrus the Great unleashed a plan which diverted the Euphrates further upstream, lowering the water level so that anyone could walk through it. His army snuck under the river's metal gates under the cover of darkness and took the city from within. While capable of stopping any direct means of assault, the walls couldn't stop one man's determined ingenuity.
Much of the ancient walls still stand, a testament to their strength. In 1983, Saddam Hussein began a reconstruction project at the site of the ancient city, including restoration of the Walls and a recreation of the Ishtar Gate. The project has since been put on hold, but Iraq hopes to continue further restoration of the walls and once again make Babylon a place of wonder.