Automated plasma turret

The Automated Plasma Turret is used to stop players from going out of Halo 3 multiplayer levels. It is not in campaign. The only level that has them is Snowbound, although Sandtrap has a minefield that works the same way. They are of Covenant technology (although much of the Covenant's technology was stolen from the Forerunners) and surround the perimeter of the map.

Whenever a player attempts to get outside the level, they are gunned down by the automated turrets. It is also still unknown why these types of turrets haven't been encountered yet. They function towards the same end as an invisible wall, to keep players within the believable confines of the map. So far, the only things that can block a turret's rounds are objects added through Forge.

The turret's rounds fire through Bubble Shields and Deployable Covers, and will still hurt players when Invincibility is turned on or if they have overshields, a behavior similar to the Brute Landmine. In terms of story value, these turrets may have been set up by the Covenant to protect the base from intruders. They would be effective at mowing through enemy infantry, especially against the Flood.

Activation
The turrets use motion trackers, to know when enemies are approaching them. These devices are presumably used to prevent intruders to get inside the fortress, as their motion trackers are pointing outwards. In gameplay, the turrets are activated when a player goes outside from the map (and the combat, too), as the players respawn inside the detection limits. Strangely, the turrets seem to have a main control console inside each of the two bases. One is turned on, but you cannot enter it.

Trivia

 * The Automated Turret may be related to the Heavy Plasma Turret, and may be an advanced version.
 * The design is extremely similar to the Shades in Halo Combat Evolved.
 * The Turrets are completely indestructible, but an inventive use of barriers to stop their fire nearly doubles the usable area of Snowbound.
 * The player can overload maps with these types of "Guardians" to make them disappear or to not fire at you, making it easier to glitch out of the maps. After a while, they will reappear and track you, but will not fire.
 * Because of its purpose, even Bubble Shields will not protect you from Automated Turrets, although scenery objects block the shots easily.
 * Strangely enough, if you go to the map in Theater mode, the alarms will still sound if you fly beyond the turrets. Obviously, they do not shoot at you.
 * Even if you make yourself invulnerable to damage the turrets will still kill you.
 * The color of the plasma shot by these turrets is similar, if not the same as the Phantom's.
 * The turrets' firing sounds are similar to the Phantom's.