From Halopedia, the Halo wiki
The Forerunners constructed massive plug locks that are required to be disengaged in order to deactivate the containment shield in the Sentinel Wall. All four plug locks have to be disengaged or the shield remains activated. The plug locks are well defended by Sentinels and Enforcers to keep the locks from being disengaged beyond orders. The plug locks may be disengaged by small arms fire, which occurred during the Battle of Installation 05.