Citadel

Citadel is an upcoming Halo 3 map that will be included in the Mythic Map Pack. Little is known about the map but it's name may suggest ties to a building similar to the Citadel in the Ark on the campaign level Covenant, not on the Ark level.

Skull
The crest of the hidden skull on this level appears to have some sort of a Forerunner symbol on it.

Trivia

 * This map is expected to be released with Assembly, Sandbox, Orbital, Longshore, and Heretic in the Mythic Map Pack.
 * This map contains a hidden skull. Locating the skull earns the player the 'Citadel Skull' achievement.
 * Getting a Sword Spree medal on this map earns the player the 'Blades of Fury' achievement.