Citadel

"Citadel may be a long abandoned Forerunner stronghold, but its small, symmetrical confines are far from secure. Two opposing bases offer little safety for players looking for solace. Instead, Citadel forces small squads to stay in constant communication, keep their heads on a swivel, and be prepared to adjust combat tactics in close quarters."

- Bungie

Citadel is an upcoming Halo 3 map that will be shipped on September 22nd, 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.

Achievements
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. Another achievement for citadel is Pull, an achievement on Citadel given to the player for getting a shotgun spree, pull is worth 25 Gamerscore.

Trivia

 * The location seems to be set either inside the Ark's Cartographer or an area similar to it.