This is the final Dark City level. Its counterparts are Starlight Cradle: Torporal Titan and Concrete Jungle: Briar Bone Barrage.
Fight through several skirmishes. Trojans spawn for the last fight. The soundtrack changes from the standard Dark City soundtracks to "Spiral Knights Main Theme" for most of the level as soon as the first party button is activated.
The 8-1 Mission: The Return of Ur always has this level as its final combat floor.