Crow's Nest

Holdout is believed to be the second campaign mission of Halo 3.

It takes place in a UNSC Base. It is the possible location presented at the end of brute vidoc, and is probably the location seen  seen at 00:45 to 00:47 and 00:54 in the E3 2007 trailer, showing a few Mongooses travelling through it and the Master Chief and Arbiter defending it with AIE-486H Heavy Machine Guns. At approximately 1:02 in the trailer, Drones can be seen picking up marines somewhere in the base, as well. Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn the "Holdout" achievement and 20 Gamerpoints.