- "We hit these three generators, and the barrier will fall?"
- — Commander Miranda Keyes
Three of these structures surrounded the Citadel on Installation 00, projecting an energy shield that denied access to the Citadel. They could be individually deactivated by means of a holographic control panel on their upper floors. Multiple similar structures were mounted on Installation 05's Sentinel Wall, surrounding the Quarantine Zone around the Library.
Installation 05 possessed a series of barrier towers placed on top of the Library's Sentinel Wall, projecting a shield which can be deactivated manually from within the wall. Superficially, the towers on the Ark somewhat resemble the Cartographer on the same structure, although they are smaller in size. The beam required to sustain the shield is projected from both the top of the tower's spire and from the bottom.
Like other Forerunner architecture, the towers can be built on a variety of landscapes, as demonstrated by the differences in the towers on Installation 00. The first tower encountered was set in a small valley and had an access tunnel that penetrated through the rock. The second barrier tower was seen to be built in the side of a cliff, and had no entrances other than the ones that led directly into the central room. The final barrier tower was shown to be situated on a small plot of land with ramps that either vehicles or infantry could traverse.
The towers on Installation 00 also had an observation platform jutting out over them. Inside the control room, there was a large glass window that overlooked the Citadel, surrounded by a semicircular formation of the towers. Terminals were also shown to be put inside some. The shield that the towers projected was shown to be strong enough to absorb plasma torpedoes fired from a CAS-class assault carrier. In or around a barrier tower, there is a cavern or tunnel that allows access behind the perimeter. It is possible that each tower has its own power source, as deactivating three barrier towers only allowed "a small section" of the overall shield to fall.
Battle of Installation 00
- Main article: Battle of Installation 00
During the battle of Installation 00 in late 2552 between the United Nations Space Command/Sangheili alliance and the Covenant forces loyal to the Prophet of Truth, the UNSC and the Sangheili sought access to the Citadel, but were barred by means of the energy shield of the three barrier towers. Commander Miranda Keyes coordinated a simultaneous assault on all three towers, ordering SPARTAN-117 and a small Orbital Drop Shock Trooper and Marine force to take the first east most barrier tower, the Arbiter and the Sangheili to take the middle barrier tower, and for Avery J. Johnson and a Marine detachment to assault the west most barrier tower.
Each tower was heavily protected. The tower which John-117 was assigned to take out was protected by an Anti-Aircraft Wraith and a Jiralhanae War Chieftain as well as multiple Shade turrets, a Wraith, and a Chieftain guarding the controls. Johnson's tower was protected by two Anti-Aircraft Wraiths, one normal Wraith, a War Chieftain lookout, a pair of Mgalekgolo and a swarm of Yanme'e. The controls were protected by Jiralhanae Stalkers and a War Chieftain.
While SPARTAN-117's and the Arbiter's groups managed to deactivate their two assigned barrier towers, Johnson's force ran into heavy Covenant resistance at the third tower, and Johnson's men were killed while Johnson was taken prisoner by the Covenant and taken to the Citadel itself. John-117 was ordered to take a Hornet VTOL to attack and land on the third barrier tower, deactivate it and attempt to find Johnson. Even though the Spartan successfully deactivated the last barrier tower, he was unable to find Johnson. Immediately after the third tower's deactivation, the Flood-infested High Charity appeared and crashed on the Ark's surface nearby. As it went down, it released large chunks of debris, as well as Flood dispersal pods, one of which smashed through the window of the tower's control room, allowing numerous combat forms and infection forms to reach the tower and its exterior.
- The Thunderstorm skull on Halo 3 can be found on an exterior balcony on the middle barrier tower where the Arbiter and his Elites go in the start of the level, The Covenant.
- All three of the Forerunner Terminals found on the level The Covenant lie within barrier towers.
List of appearances