From Halopedia, the Halo wiki
Forerunner Power Node
Power resource point
The Forerunners built the Ark to be completely self-sustaining, and much of its power lies in the vast subterranean web of power structures beneath its surface. These external power nodes were designed to provide auxiliary power during repairs or emergencies, giving those in control of them a steady flow of power resources.
On this remote installation, light years from any support, both the UNSC and the Banished have realized the strategic importance of Forerunner power nodes. Eschewing the Covenant's reverence for Forerunner artifacts, Atriox has no compunction of taking control of them for his own ends, and many a battle involves a desperate scramble to capture power nodes before the enemy can.
Forerunner sentinels that can be called in by Anders to cause area denial.
Though their primary purpose is to maintain, repair and protect the Ark, these Forerunner Sentinels can be summoned by professor Anders, who has found a way to deploy them in the service of the UNSC. Once called, the Sentinels swarm into a targeted location and attack any hostiles in the area with lasers beams, remaining and causing area denial for a duration before returning to their jobs on the Ark. Any Sentinels that are destroyed emit an EMP blast, crippling hostile vehicles temporarily.
The Sentinels are worker drones with basic AI, created by the Forerunners to safeguard their stations as well as perform repairs and maintenance. There is a large variety of Sentinels in both purpose, size and appearance. There are small Constructor drones used to maintain Forerunner structures, huge Retrievers used for mining and Aggressor drones for attacking any perceived threat. The Sentinels will only attack if they perceive a present threat; otherwise they will happily ignore strangers and carry on with their work.
Forerunner Control Tower
Multi-purpose control structures
As the Ark's name suggests, it was designed to play host to an incredible variety of life collected from across the galaxy. This almost inconceivable volume of biodiversity necessitated a flexible environmental system on the Ark as the Forerunners prepared for the galactic reseeding of life after the Halo Array was activated.
On a practical level this meant that all habitable surface area on the Ark could be covered in an environment to match any life-sustaining planet in the galaxy.
Atmospheres, soil and vegetation makeups, bodies of water or other liquids, and even gravitational conditions could be custom-set in areas as large or as small as needed to sustain collected specimens from a particular planet.
These environmental settings were accomplished through a network of Control Towers. These towers worked essentially like the output nodes of modern mass-particle replicators or the archaic 3D printers of the 22nd century, generating kilometers-deep environments around them through manipulation of raw materials stored deep within the superstructure of the Ark. The base of each tower then generated desired gravitational conditions.
When an environment was complete, only a small percentage of each Control Tower remained visible above ground in order to minimize interference with the new local wildlife while still allowing for data gathering on the surrounding area.
Today, these Control Towers lie mostly dormant. But with new visitors to the Ark and the possibility of new uses for the Ark's scientific and military capabilities, the towers offer literally world-building power to whoever controls this ancient installation.
Forerunner defense constructs controlled by Professor Anders.
Exclusive to Professor Anders, the Protector Sentinel is a flying Forerunner drone armed with an energy-based sentinel beam that is effective against all enemies. It can be upgraded to a sentinel swarm, which adds smaller Aggressor drones as an escort. Sentinels explode in an EMP blast upon death, stunning nearby enemy vehicles.
Professor Anders has shown a remarkable ability to manipulate Forerunner command codes and bypass directive locks, giving her the ability to bind the Ark's own defense constructs to her will. The full extent of her capabilities have yet to be tested, as the Professor's attention is constantly diverted to more pressing needs like finding a way home.
Extremely powerful Forerunner drone
The Ark's Retriever Sentinels are among the most powerful Forerunner drones, their primary purpose being the mining of the Foundry in the creation of Halos and Ark maintenance. However, like all Forerunner drones, they can also be used to defend the Ark by turning its sentinel lasers on anyone who threatens it.
The Retriever Sentinels played an important part in repairing the Ark after the events of 2553 in the final battle between the UNSC and the Covenant. UNSC scientists at the Henry Lamb Research outpost reactivated the dormant retrievers and watched in awe as the Ark was restored to its former glories. Sadly, war has once again come to the Ark, but the Forerunner drones built to protect it will stand fast and push back against any and all invaders.
Forerunner system offensive/defensive drones
The Controller Sentinels’ primary function is to guard valuable Forerunner technologies on the Ark. Developed and deployed to counter the escalating threat from the UNSC and the Banished, these flying Sentinels are loosely 'tethered' within set radii of important sites and will attack anyone that comes close, or attempts to take control of the sector.
An impenetrable energy shield protects the front of each Controller Sentinel, an improvement on the already powerful shields used by the Enforcer Sentinels whose function was to suppress Flood outbreaks. Underneath it fires a continuous Sentinel Beam to prevent enemies from hiding in its blind spot, but its primary weapon is a devastating supercharged Sentinel Pulse.
A long dormant Despair-class fighter
Forerunner Despair-class fighters saw action during the Forerunner-Flood war against the rogue Monitor Mendicant Bias. After the firing of the Halo superweapon, some of these fighters moved to Shield Worlds and Halo installations.
The Forerunner ship found and reactivated by Colony's Huragoks was 'regrown' from a ship-seed planted by the Librarian prior to the Halo activation. Ship-seed technology provides all the building blueprints needed to built a ship upon activation using on-site materials.
This Forerunner structure is a vital node in the Ark's defences and must be preserved at all costs.
After the Halo Array eradicated the Flood threat and the Forerunners disappeared, the Ark lay dormant and untouched for millennia.
When the Human-Covenant war spilled over onto the Ark, the installation's threat assessment systems began an exhaustive recording and indexing of events in an effort to anticipate dangers and prepare adequate countermeasures to deal with similar challenges in the future.
These threat assessments compare modern battle records against all catalogued historical conflicts, cross-referencing them with updated information and calculating probable outcomes. The systems performing these assessments are powered by the Terminus structures, the loss of which could result in the corruption of centuries of analysis, compromising the Ark's long-term ability to defend itself.