Constructor Sentinel

Constructors are specialized Sentinels built by the Forerunners. Their only function is to repair Forerunner structures that appear to be damaged. Aggressor Sentinels will come to their aid if they are attacked, as they have no weapon systems of their own to defend themselves.

The Construction Beam is fired from the top half of the Constructor, however the constructors on The Ark fire their beams from the bottom. Constructors also play another role as a security grid. When a Constructor is attacked, an unknown signal causes the Sentinel launchers to begin deploying Aggressors. Constructors form an excellent security perimeter.

Design details
Constructors are drones that mainly repair damage to any Forerunner structure they can find, using weak energy beams similar to Sentinel beams. The beams are small and blue, and are harmful if they come into contact with an organism. It is unknown whether they will harm another Constructor.

Aggressors will come to a Constructor's aid if it is being attacked, as Constructors are ill-suited for combat, having no weapon systems of their own. However, their beams can cause damage to the player if he gets caught in its path (on Legendary difficulty their beams can kill in seconds). They also use their beams to lower the shields on Pistons, allowing them to move throughout the Sentinel Wall quickly.

Gameplay
In Halo 2, they first appear in Sacred Icon, exactly beside where Arbiter stands after he was dropped off from his Phantom. They were repairing the destroyed edge and left after a while, or if the Arbiter approaches them. Attacking them will result in being attacked by Aggressors. Then in the Quarantine Zone level, they appear at the end of the level when the player fights Flood-controlled Scorpion and Wraith tanks along with many Flood using Covenant Ghosts and a Spectre, and two Enforcers fight against the Flood near a downed UNSC Pelican dropship. During all this chaos, if the player watches one of the large and deep trenches, a large cloud of Constructors can be seen flying off into the sky.

In Halo 3, Constructors appear in some levels based on the Ark. In the campaign, they appear in the level The Ark, where they are seen inside Forerunner structures including inside the wall to the Cartographer and the map room itself. In multiplayer maps Isolation and Construct, small groups of Constructors can be seen flying around in the distance. Also, in Halo 3, a small easter egg can be witnessed if a player watches one of the groups of Constructors on the multiplayer map Construct. They will fly normally for a while, but when they near a wall, they will hop like bunnies, before passing out of the map, and to a destination unknown, possibly re-spawning on the other side. You can destroy them and the Aggressors in the level.

Trivia

 * Aggressor Sentinels have been seen using the Construction beam in the level The Arbiter.
 * In the Halo 2 level Sacred Icon, Constructors will open the Pistons for you if they are near them.
 * On the level Sacred Icon, where you find the Grunt and the Jackal, if you look off the side into the chasm you will see an endless stream of Constructors coming out of a bright hole in the wall.
 * On Isolation and Construct the Constructors can be sniped, though their small size and constant movement make this difficult.
 * In Halo 2 and Halo 3, even though Constructors do not attack you, your weapon reticule will turn red when passed over them.
 * A player can get up close and even touch a Constructor on Construct. First he/she must overload the map with trip mines. When the map is overloaded the player must go through the hole where the purple lift beams are projected. A player then can easy be touched by a Constructor but cannot be moved. They can be destroyed, mainly with a Spartan Laser or a Flamethrower. Note that even when one is destroyed another will immediately replaces it in the same place. So in other words, you will see the debris from a destroyed Constructor fall from a full and complete one.
 * In Halo 2, the Constructor's beam light is on the top part of its structure, whereas in Halo 3 the light is on the bottom half.
 * Constructors appear to be able to compact into a smaller form, as their top and bottom halves appear to latch together. However, this has yet to be seen.

Appearances

 * Halo 2
 * Halo 3