The Broken Frontline is a ring of defenses around the DisSea which defends DisCity from BR-000. After the DisSea Explosion, the Framework Against Catastrophe was founded to protect the City from all Mania-related threats. One of the FAC's first tasks, and one which remains among their primary duties to this day, was the construction and defense of the Broken Frontline. The task of bringing BR-000 under control took five years; though it is unclear when in this time period the Broken Frontline was built, its completion can be reasonably assumed to be close to N.F. 89.
The layout of the defenses are unclear, but possibly a set of concentric circles of fortresses. During both the first descent of BR-002 and the appearance of Dreya, FAC forces were said to be pushed all the way back to the Third Broken Frontline, which may be the final line of defense before DisCity. Due to the span of eight years between these events, it seems that the FAC is able to reclaim defeated Frontlines after crises are resolved, but this may either be a natural process as fluctuating contamination recedes or a protracted conflict to drive out the monsters that overran them.
The Broken Frontline's base station is dominated by a shield system, which projects force fields that hide the DisSea from view. This is a necessary defensive measure, as just seeing or knowing about the existence of a Black Ring is enough to cause mental contamination in an untrained human. However, this need for secrecy also breeds suspicion and conspiratorial thinking. As seen in Stargazer's interrogation, there are some who believe that the Broken Frontline exists to hide the government's secret Hypercube stockpile. Thus, while the FAC's main duty is to defend the Frontline against monsters from inside the DisSea, they must also occasionally defend it from raiders misguidedly attempting to break into the DisSea from outside.
In addition to its defensive fortresses, the Broken Frontline included many monitoring stations built in the DisSea's shallows to inspect the condition of the Black Ring and research its contamination. However, as contamination within the region continued to worsen over the course of years, attacks from Mania-created monsters increased, and the Frontline's monitoring and research functions took backseat to combat and defense. These monitoring stations became the foundation for the impregnable fortifications built on top of and around them.