
Note that the soldiers are received in order, beginning with the top 3.It is best to leave these top soldiers fully equipped between missions after completing the Alien Base Assault players who frequently use the loadout command "Make all items available" should be especially wary. The top 5-6 soldiers available from the Barracks are automatically assigned to the mission the player receives them equipped with whatever they had when the mission starts. The player cannot choose which soldiers are used during the mission, nor their loadout.The Ethereal can potentially be introduced here instead of the Overseer Crash Site. The mix of aliens during base defense is not normalized, meaning that a player may face an "easier" mix with more Sectoid thrown in, or a harder mix with stronger aliens. It is possible for this mission to feature every alien type with the exclusion of Outsiders. The alien force is randomized again when a player restarts the mission after failure, meaning that each attempt at base defense plays out differently. The exact composition of the alien force that the player faces is mostly randomized (as well as some of the alien spawn points). The number (as many as 44 on Normal and Classic) and type of enemies vary depending on the stage of the alien invasion, similar to the Alien Base Assault. Together they form an elongated rectangular shape consisting of a Rear Guard Station, Command Center, Mech Bay, Forward Access Tunnel, and Forward Guard Station.

The areas of Delta Section are identified as alien reinforcements arrive throughout the mission. This mission takes place on a unique map detailing the expansive XCOM HQ Delta Section, an area of the XCOM headquarters located behind Mission Control. ▪ If all soldiers in the combat area are killed, XCOM HQ will fall Limited reinforcements, including XCOM Base Security personnel, are available. The invaders have infiltrated XCOM HQ in unknown numbers. – Central Officer Bradford, XCOM: Enemy Within We have to protect the base at all costs - if we don't hold the line here, any hope we had of stopping this invasion will be lost.

Hostile forces have infiltrated XCOM headquarters and are moving in with reinforcements. The mission is launched immediately following the cutscene, without an opportunity for the player to choose and equip the squad members. While in Mission Control the Hologlobe will begin to behave erratically, followed by a cutscene where multiple XCOM personnel, under the influence of alien Mind Control, commit acts of sabotage and violence. This mission occurs 10-20 days following the Alien Base Assault mission.
