From Halopedia, the Halo wiki
Sentinel walls are massive walls on Halo installations intended to serve as means of containment if primary measures fail in the event of a severe Flood outbreak. As their name suggests, the walls house numerous Sentinels to both repair the facility and keep the Flood from spreading beyond the wall; they also serve the function of generating an energy barrier around the Library. A Sentinel wall surrounded the Quarantine zone around the Library of Installation 05 to keep the Flood from spreading.
The wall appears to have been designed to keep the outbreak of Flood around Delta Halo's Library contained within the Quarantine Zone. It is honeycombed with 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. In addition to the massive physical wall, the Sentinel Wall also projected an energy barrier known as the containment shield. This shield could only be deactivated from a console deep within the wall's interior, locked in place by a series of plug locks.
Battle of Installation 05
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. During the battle on Installation 05, the Arbiter was sent into the wall to deactivate the shields keeping the Flood in the Quarantine Zone in order to allow other Covenant forces fly over the wall towards the Library.
List of appearances