A gigantic structure situated outside of any known star system, the Ark was constructed by the Forerunners with two guiding principles. First, to fabricate, repair and transport all components of the ringworld installations known collectively as Halo, a weapon designed to wipe out all sentient life in the galaxy in a desperate attempt to stop the unstoppable parasitic Flood threat. Secondly, it needed to be able to provide a safe and stable habitat for a vast number of life forms collected from across the galaxy.
These two paradigms led to the unique, flower-like design, with immense arms splayed around a central foundry. Where the production of the Halo installations occurs. The Ark's surface was covered by vast, world-sized paddocks called Refugia, divided by massive walls and containment barriers to preserve the life forms collected by The Librarian, to be reseeded after the Flood had been starved out.