Shield worlds, also known simply as Shields, are Forerunner installations designed as a defensive strategy against the Flood and an alternative to the Halo Array. As opposed to sterilizing the whole galaxy at once, shield worlds would act as a more precise and conventional solution, serving as both refuges for local populations and powerful military fortresses from which to carry out strategic campaigns against the Flood. Several varieties of shield worlds were designed over nearly nine thousand years; in some iterations the interior was suspended in fractal slipspace, protecting organisms within from the Halos' destructive pulse.
Shield worlds were used by the Forerunners as military outposts for thousands of years before the Forerunner-Flood war. After the Forerunners learned of the parasite known as the Flood in the later stages of the war with the humans, they began the construction of more shield worlds. Many shield worlds were planned to be mobile, able to be deployed directly into infected systems to purge local infestations, and would be designed as facilities from which to fight and research the Flood, an alternate solution to the plan to construct the Halos. Numerous shield worlds of different configurations were built over thousands of years, but the construction of additional shield worlds was halted due to the need to progress the Halo array construction faster. Though the construction of the shield worlds was abandoned, they still found use as shelters for the various species the Forerunners indexed and collected in the Conservation Measure. Many Forerunners also intended to evacuate to the shield worlds, but the betrayal by the Forerunners' supreme defense AI Mendicant Bias and its defection to the Flood forced the Forerunners to abandon these plans in favor of the ultimate solution to the Flood: Halo.
Even though the majority of shield worlds were left empty, there were still many that remained fully stocked with different forms of Forerunner technology. These fully stocked shield worlds were the ones used in an attempt to counter the Flood infestation. As a result, these shield worlds are a treasure trove of defensive and offensive technology used by the Forerunners along with research tools and research data on the Flood. One such shield world has been found: Shield World 0156.