Deployable lookout tower

The Covenant use deployable lookout towers when they set up fortified positions to alert them of advancing forces and to give their snipers an elevated view of the the battlefield. In Halo 3, some have deployed Plasma Cannons in it manned by Heavy Grunts, for close quarters action.

Summary
They are basically a large platform suspended in the air by a medium-sized anti-gravity generator which is also used as a miniature Gravity Lift to ferry snipers and lookouts to and from the platform. In Halo 2, they appear in many Campaign levels as well as the multiplayer level Relic. The only two problems are that it's suicidal to stay there for a long time because your opponents will know where you are and you must jump out of the window in order to leave the tower.

The platforms are capable of being knocked off the gravity lifts with rockets. This is achieved by firing two rockets at the corner of the platforms, or, if available, keep firing Gauss blasts at it. It will end up falling off, allowing you to float on the gravity lift.

In Halo 3, the platforms are destructible. Players also can destroy the fins on the top of the tower before destroying the entire tower itself. An easy way to destroy the platforms is to go on top of it, and throw a Plasma grenade or Spike grenade. (This is only advisable on Easy or Normal, as it may be difficult to do on higher difficulties.) One to three Plasma Cannons will be mounted on lookout towers, manned by Heavy Grunts. Brutes will sometimes take position in the lookout towers to shoot at opponents with a Carbine or Beam Rifle, it just depends on the difficulty. There is usually a File (a small Covenant ground infantry unit) stationed with the tower to help support the Grunt Heavies in case of attack.

In Halo Wars these buildings can garrison in units from either player increasing their combat capabilities.

Glitch
On all Halo 2 gravity towers that have been knocked down, the base is still active and can be used to push you up directly into the air. From this position, you can continue to hover and snipe enemies. This glitch is absent from Halo 3; once the tower is destroyed, the gravity lift of the base does not work. However, the tower itself (if still intact) will push you through the hole in the center, as if the gravity lift was under it.

Trivia

 * Halo 2 is the only game in the trilogy to use Deployable Lookout Towers in multiplayer. Halo 3 may contain Deployable Lookout Towers in multiplayer in future map pack releases or as Forge items, but this is unlikely.
 * With the length of the tower, 9-2= 7, quite possibly another 7 reference.