Sentinel

Sentinels are autonomous machines created by the Forerunners, designed for the construction, maintenance and protection of their structures and facilities. Various Sentinel variants exist, each tailored to a specific task; because of its ubiquity, the Aggressor variant is often referred to simply as the "Sentinel". All variants are produced by Sentinel manufacturing facilities.

Aggressors
The Aggressor is the most common form of Sentinel, designed for the protection of Forerunner constructs and the small-scale suppression of Flood forms. There are many forms of Aggressor, though most follow the same basic layout. In the event of a severe threat, an advanced version of the Aggressor, known as the Sentinel Major, would be dispatched to assist the more common Aggressors.

Assembler
Assemblers are presumably used to build and maintain Forerunner installations and technology, like the similarly named Constructors. Though they are not designed for combat, they will suspend their primary duties and engage Flood forms in the event of an outbreak.

Constructor
The Constructors are specialized Sentinels, designed solely for the construction and servicing of Forerunner structures. Like Assemblers, they are not designed for combat, but they will engage the Flood if necessary.

Enforcer
The Enforcers are equipped with a variety of offensive and defensive measures, including pulse beams, missiles, a last-resort Sentinel beam, and three frontal energy shields. They are designed to handle major Flood outbreaks, but are also capable of defending their Installation from other external threats.

Protector
Roughly similar in appearance to regular Aggressors, Protectors are modular units capable of being modified into one of three variants: Offensive, which is a weaker version of the standard Aggressor; Shield, which projects an energy shield around its host unit; and Healing, which provides medical assistance to its host after combat.

Retriever
The Retrievers, otherwise known as the Strato-Sentinels, are massive terraforming machines, used to gather raw materials for large-scale Forerunner astroengineering projects such as the Halo Array.

Watcher
The Watcher is a Sentinel variant used by Promethean Knights for combat support. They can heal and augment their damaged allies and return incoming ordnance such as grenades using a gravity-displacement beam.

Other types
The following machines have either not been conclusively identified as Sentinels, or have not been given an official designation other than their common label.

Council Sentinel
A special variant of Sentinel was used aboard the Forerunner capital for security purposes. They possessed black armor, matching the black-and-red armor of Ecumene Council security personnel.

Gatherer
Gatherers seem to be deployed exclusively on Line Installations, and serve as collectors of biological specimens for the resident Monitor. They have offensive capabilities similar to Aggressors.

Guardian Sentinel
Little is known about this variant's specific purpose. The Guardian Sentinel has a single red "eye", which fires a significantly more powerful beam than the standard Aggressor.

Onyx Sentinel
Onyx Sentinels both guard and make up the artificial planet of Onyx. With a much more powerful individual energy beam, as well as the ability to combine and transform themselves in order to complete a specific task. Onyx Sentinels are far more formidable opponents than their more standard variants.

Super Sentinel
Considerably larger than Aggressors, Super Sentinels fill the same role as the Enforcers - combating large-scale Flood outbreaks.

Unidentified Sentinel variant
The largest Sentinel variant seen to date, this Sentinel is able to level entire cities with its weaponry.