Foundry (location)
From Halopedia, the Halo wiki
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The Core was probably originally located within the Milky Way galaxy, but was moved by the Forerunners. However, it is possible that it was an extragalactic "rogue" planet; not gravitationally bound to any stellar system.
Defences
Along with the massive Sentinel population found around the Ark, the Core has an extra, albeit passive defence. It is equipped with a barrier shield which can be raised to protect both the Core and the Foundry (the industrial infrastructure which supports the creation of replacement Halos) from attack by Flood. At least one superluminal communications array (read: faster-than-light radio) is also included within the barrier's confines.
The Covenant Prophet of Truth raised this barrier in 2552, either intentionally or accidentally. A small section of it is subsequently lowered by a joint UNSC-Covenant Separatist assault force in order to allow access to the Citadel.
