From Halopedia, the Halo wiki
Spires are Covenant structures that serve as bases of operation during ground engagements. They function as teleportation hubs for invading forces, and are also capable of projecting energy shields over a large radius. Covenant forces utilize these teleportation spires during large-scale operations.
The spire consists of multiple sections: a lower section, which projects several energy beams that keep an upper section suspended high above it as well as powering a floating shield-generator array. From the upper level, a large dome-like energy shield is extended to the ground. This shield is resistant to conventional weapons, and is capable of disabling any enemy vehicles that pass through it.
One of the main functions of the spires is to teleport ground forces to the surface from a large vessel such as a supercarrier. Troops teleporting through the spire commonly utilize an air vehicle, generally a Phantom dropship, as the objects being teleported will materialize in the space between the upper and lower main sections of the structure. This technology is loosely modeled after the Forerunners' translocation system.
There are two ways to reach the top of the Spire: via the same gravity lifts that hold the upper level aloft, or off one of several lifts positioned at intervals around the Spire.
Unlike most other forward operations structure and vehicles, Spires do not have an easily accessible reactor core like Type-47B Scarabs or Type-38 Anti-Aircraft Cannons. The shields generated by the Spire must be deactivated manually through a control interface in the upper portion of the structure.
Covenant spires are presumably deployed from starships and dropped directly onto their intended location; this is evidenced by at least the nature of the only Spire known to be ever observed at close range, Spire One. The Spire's base lay partially buried at the center of a massive crater, surrounded by blasted sand. This may have resulted from it being dropped from a great height.
Use in combat
Covenant Spires are used primarily as command posts and teleportation sites for troops. Their shields make them difficult to attack, as they can only be disabled by ground forces after they infiltrate the structure. They also presumably make effective watch-towers.
Multiple Spires were seen during the Covenant invasion of Reach. Covenant ground forces deployed these behemoth structures to cloak the presence of a Covenant supercarrier, Long Night of Solace, flagship of the Fleet of Valiant Prudence. One of these Spires were disabled and destroyed in a UNSC counter-offensive operation.
List of appearances