Citadel

Citadel is an upcoming Halo 3 map that will be shipped in Fall 2009 with Halo 3: ODST. One could believe that this level resembles a forerunner citadel, much like that of The Library in Halo 1, based off of the name, and the Shotgun Spree achievement that is earnable here. The shotgun can be seen as the human weapon most associated with the flood.

Skull
This map has a skull for the multiplayer 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.