Barrier tower

A Barrier Tower is a Forerunner shield-generation tower.

At least three of them 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 Barrier Towers also surrounded Installation 05's Quarantine Zone, mounted on top of the Sentinel Wall.

Battle of Installation 00
During the battle on Installation 00 in late 2552 between the United Nations Space Command/Covenant Separatists alliance and the Covenant Loyalists, the UNSC and the Elites 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 eastmost Barrier Tower, the Arbiter and the Elites to take the middle Barrier Tower, and for Avery J. Johnson and a Marine detachment to assault the westmost Barrier Tower.

While SPARTAN-117's group and the Arbiter's group managed to deactivate their two assigned Barrier Towers, Johnson's force ran into heavy Loyalist resistance at the third tower, and Johnson's men were murdered as Johnson himself was taken prisoner by the Brutes and taken to the Citadel itself. SPARTAN-117 was ordered to take a Hornet VTOL and attack and land on the third Barrier Tower and deactivate it and attempt to find Johnson. Even though SPARTAN-117 successfully deactivated the last Barrier Tower, he was unable to find Johnson.

Afterwards, the shield around the Citadel was lowered and the UNSC Marine Corps launched an all-out assault on the Brute-held Citadel.

Trivia

 * The "Thunderstorm" skull of Halo 3 can be found on an exterior balcony on the second Barrier Tower.