The Rampart of Babylon (バビロンの「城壁」,Babiron no "Jōheki"?) is one of the nine legacies left behind by the genius doctor Regina Babylon. It was created approximately 5000 years ago during the age of the ancient Partheno Civilization. It was later discovered by Touya Mochizuki and is managed by the gynoid Preliora. It is the fifth Babylon to be discovered and had already docked with the Tower.
The Rampart of Babylon serves as the defense system for the combined Babylon. It is capable of defending against physical and magical attacks by deploying a protective shield against them. It can defend against airborne enemies by deploying a weapon known as the Satellite Orb against them, it is capable of tracking and intercepting targets automatically when activated. There are 24 Satellite Orbs stored inside of the Rampart that surround it like actual satellites when necessary.  The Rampart's defenses are controlled through a terminal inside a small castle that serves as the physical form for The Rampart. Only Touya Mochizuki and Preliora can control the defense systems by using a monolith found within the castle itself. There are also 15 mini robots located in the main castle that are capable of performing simple tasks when ordered.
The teleportation circle for the Rampart is located in a ruin located in the southern region of the Knight Kingdom of Restia. 
The Rampart is one of only two pieces of Babylon that are capable of self docking the other being the Tower.
Both the Rampart and the Tower had a chance encounter 153 years before their discovery and was able to achieve docking due to the Tower's teleportation circle being destroyed 374 years prior.
The Satellite Orb would become the basis for the Fragarch weapon system utilized by the Frame Gears.
The defensive shield produced by the Rampart would become the foundation for the production of the Bracelet of Defense (an artifact created by Regina Babylon) that was used General Bazur during the attempted coup of the Regulus Empire.
Despite the vagueness of the discription, the Rampart is a full castle; which is where Melle stayed safely sealed away until Touya placed a divinity-enhanced seal around her core to block its sound from the Phrase.