The Foundry is a massive circular machine situated at the epicenter of the Ark for the purpose of constructing and repairing the ringworlds that make up the Halo Array.
Within its center it holds a mineral-rich moon in a powerful gravitational field that is mined for resources by giant Retriever Sentinels, then used by Assembler Sentinels to build components of a Halo. The Halo Array is always made of seven ringworlds and should one be destroyed the Ark starts building a replacement.
Once a Halo is stable, it is released to the same location as the destroyed Halo to replace it while Sentinels continue to work on it. Of the seven ringworlds that make up the Halo Array, six have originated on the Ark, while the seventh was constructed on an earlier version of the Ark and later reconstructed to fit the new design.