Citadel

Citadel is an upcoming Halo 3 map that will be shipped in Fall 2009 with Halo 3: ODST. The Shotgun Spree achievement is attainable on this map. It is possible that this map is located in a Forerunner Citadel, similar to the one seen on The Covenant.

Speculation
On the MLG Forums, a member named NeonSurge claimed he had a chance to play this map and the other maps, Heretic and Longshore. On his post, he commented on Citadel saying:

"I played this map with Lars. This map was LARGELY inspired by the level "The Covenant" when you were inside of the Forerunner structures, right after the battle with the Hunters who were below the elevator platform. When you have to go around and up an angled jump to battle a few brutes and buggers to get to the platform, that part is almost exactly in the map! It was really cool and was a good close quarters map. Two overshields spawn, and there is a central location with a small structure in the middle with Rockets on top. There are a lot of things hidden in here, very cool map."

Skull
This map has a Skull for the Multi-player Achievements. When found it unlocks the achievement Citadel Skull. It has an image on the skull that resembles a Forerunner glyph, indicating that it may take place on Forerunner terrain or, given the Covenant's use of Forerunner glyphs, is Covenant in aesthetics.