A Shield World or Shield Installation is a Forerunner installation that is designed to be a shelter from the activation of the Halo Array. Each Shield World encountered to date has had significant differences in their design. Presumably, each was constructed for different purposes. Access to Shield Worlds is only possible upon the imminent activation of the Halo Array  but have been known to open on other occasions such as a Reclaimer being present.
Each Shield World is different in design, but they all share some common features. They have a planet-sized exterior shell, often with some amount of terraforming, presumably to act as camouflage. They all have Sentinel systems for protecting and maintaining the facility. Most have an extensive amount of Forerunner facilities, either above or underground. Finally, each serves the same basic purpose: as a "bomb shelter" from the Halo installations, while a few rare ones, like Requiem, work as a fortress for the Forerunners.
The Shield Worlds were built by the Forerunners in their vast empire, as refuges from the Halo Array's pulse. However, the betrayal by the Forerunner AI 032 Mendicant Bias and its defection to the Flood forced the Forerunners to abandon their plans to evacuate into the shield installations, as Mendicant Bias revealed the locations of the Shield Worlds to the Flood.
However, as some Forerunners survived the activation of the Halo Array, including Bornstellar-Makes-Eternal-Lasting, seeding again the galaxy with life before departing forever, they must have been on Installation 00 or in some Shield World .
Caches of Technology
All Shield Worlds act as "bomb shelters." Some Shield Worlds act as a sort of protected armory, rather than as living quarters. For instance, Shield 0459 contains Forerunner starships, among other pieces of technology. At least one also contained old experimental weapons, as Sharquoi.
The Covenant owes their rapid technological advancement to the discovery of a small number of these worlds, discovered earlier in their history and stripped of useful resources. The fate of these worlds, and whether they were abandoned or inhabited by the Covenant, is unknown.
Known Shield Worlds
Trevelyan (Onyx/Shield 0006)
- Main article: Trevelyan
The main component of the Shield World is a Micro Dyson Sphere, named The Sharpened Shield, which is located within a Slipspace bubble of compressed space-time. In normal space, the bubble has a diameter of only a few meters. The only way to access the bubble is a small portal in real-space, the Core Room Antechamber. It is located at the center of the artificial planet.
The Dyson Sphere within the slipspace rift is two astronomical units, i.e. 300 million kilometers in diameter, and its inner surface is entirely terraformed and capable of supporting life. In the center is a (presumably) artificial star.
It was renamed Trevelyan in honor of SPARTAN Kurt-051, who sacrificed his life so others could enter the shield world, by request of Chief Mendez. Trevelyan was Kurt's surname before he was abducted for the SPARTAN program.
It was called the "Sarcophagus" by the Forerunners.
- Main article: Shield 0459
The artificial planet is hollow, with an interior diameter of approximately 3000 kilometers. Inside is a miniature artificial sun. Its appears to function as a housing for a fleet of Forerunner Ships and other technologies in addition to being a shelter from the Halo Array's pulse. It was blown by UNSC Spirit of Fire's crew to prevent it falling on Covenant hands. The remnants were later destroyed too.
Requiem (Shield 0001)
- Main article: Requiem
- “Instead of surrounding a star, the massive exterior shell of Requiem encases a planet - a variation on a Forerunner Shield World.”
Seen in multiple trailers of Halo 4 and the legendary ending of Halo 3, the interior has been terraformed, complete with clouds and greenery and was one of the first Shield Worlds built prior to the Human-Forerunner war. The entrance portal shown is also evidently not overly secure, with the UNSC Infinity, the remains of the UNSC Forward Unto Dawn , and Covenant forces able to pass through and deploy ground forces. Requiem is also inhabited by various Forerunner AI, which, unlike Sentinels, seem to be combat specific. Such AI include the Promethean Knight, Promethean Crawler and Promethean Watcher. While Sentinels no longer act as the main guards, they are seen on Requiem in various chambers, performing repair duties. It become battlefield between UNSC and Covenant forces, and was ultimatelty destroyed when Jul 'Mdama's send it into a sun.
Refuge (Shield 0637)
- Main article: Refuge (Shield 0637)
Main article: Genesis.
- Worlds like Onyx were referred to as "The Shield" of the Forerunner, while the Halos considered "the Sword." This echoes Cortana's line on the Halo 3 Announcement Trailer, "I am your shield, I am your sword." So far, Bungie has said nothing about a correlation between Cortana's use of the word "Shield" and the Forerunner's "Shield World."
- A "shield installation" is mentioned in the cutscene on the level "The Ark" in Halo 3. 343 Guilty Spark states that he had hypothesized that the Ark might be located within such an installation, but was proven wrong by the discovery of its location on the exterior of the galactic rim. This also brings up the possibility of many Shield Worlds existing, further assisted by the Flood-controlled Shield World, Shield 0459, that appears in Halo Wars.
- In Terminal 5 of Halo 3, there is a sentence that reads the following "If we start immediately--commence total biosphere elimination of life sustaining worlds (as indicated in the accompanying charts) and relocate evacuated populations to facilities such as those described in the Onyx project all this could be achieved in [57,1590 (+/-2,184) hours]." This is most likely a reference to either the Forerunners Shield World(s) plan or Onyx itself.
- Unlike the Halos, it is unknown how many Shield Worlds exist. When the Forerunners built the Halos, it is possible they built an accompanying shield world with each. The Halos would destroy all sentient life within a 25000 light year radius of itself while the Shield World would protect any sentient life within that region.
- Halo: Ghosts of Onyx
- Halo Encyclopedia, page 232
- Halo: Fractures - Untitled story, Page ??
- Halo Waypoint, Shield World article
- Halo Wars, Timeline
- Halo Encyclopedia
- Halo 4: The Essential Visual Guide
- Halo 3, The Ark, "I always assumed the Ark was part of a shield installation"