Sentinel wall
From Halopedia, the Halo wiki
The Sentinel Wall is a massive wall surrounding the the Quarantine Zone around the Library of Installation 05. The Wall appears to have been designed to keep the outbreak of Flood around Delta Halo's Library contained within the QZ. It is honeycombed with many tunnels for the Sentinels to travel through. Sentinels and Enforcers keep the Flood contained, while Constructors keep repairing the damage that the Flood inflict on the Wall. Despite the efforts of the robotic keepers, the Flood were released when the Arbiter penetrated the defenses and deactivated the containment shield around the wall. It is unknown if such a structure exists on other Halos. During the battle on Installation 05 the Arbiter was sent into the wall to deactivate the shields keeping the Flood in the QZ in order to let other Covenant forces fly over the wall towards the Library.
