The systems and passages are pretty old and have a large quantity of moss in the humid areas, despite that the mechanisms are still working. It takes water coming from the rivers to the north and filters it, releasing the clean water to the cities near the castle. It also served as a prison in the days when the castle's owner was alive, probably decades ago. The Castechno unit that was responsible for the prison, the CTI-017 Verdugo was still wandering and functional up to the events of the comic.
The Rats established a hideout around the sewers' main computer room, this way they could use the floodgates to difficult the entrance of any intruders along with several traps, as seen in The Sewers. The hideout hides a storage of hazardous chemical substances, which can leak into the clean water if the place is somehow flooded.
A package of jammers was stolen by The Rats from the Neo Maverick Army, and they attached the four devices in the main computer, activating it and preventing any teleport near the Iberian Castle area.
Kran sent some of his soldiers to search for the jammers, but they got lost in the sewer systems. X and Zero find some of them during their way through the sewers, and during Cat and Mice one of them almost finds the hideout. After Sewer Jailrat activated the Presset 3A, the place was set to inundate itself, including The Rats' hideout, making the hazardous chemical products stored by Ivory Labrat to leak and intoxicated the water that would go to all the cities near the castle.