The Arch, situated in the geographical centre of Apotheosis City, is not just part of the City's architecture. It is an integral part of the ships systems.

An Important Structure

The Arch, as well as being an impressive futuristic architectural structure, is a large machine directly tied in to the ships systems. As revealed by Casey-McGeever in Heroic games' first Youtube video, it recovers the free floating and inert forms of Unobtanium present in the ships atmosphere, known as dust, the main ingame currency.

Only the top third of the arch is visible, the rest is below the city's ground level. At the base of the arch, on each side at that level, are doors which will lead to mission areas inside the arch itself.
And, to quote Casey himself, "Yes, there will be a badge on the top of the arch."

Also a Key Target

Pure Unobtanium, the material used to power the ships engines, is extremely valuable, not only to the humans on board the ship. This makes the arch the target of incursions by aliens looking to increase their supply of the rare substance.