An Alamo is a highly-defensible position from which there is likely no escape. Players typically Fall Back to an Alamo as a last resort, hoping to take out as many enemies as they can before being killed. The Alamo is usually the final combat position in Defense in Depth scenarios, and is usually chosen before the mission even begins.
The term "Alamo" comes from a compound/fortress in San Antonio, TX, where a famous siege during the Texas Revolution of 1836 culminated with General Santa Anna's troops storming the building and slaughtering its occupants. 180 Texans defended the fortress against 10 to 30 times as many Mexican soldiers for a whole 13 days.
Suitable Alamo sites include large buildings - especially construction sites - as well as large Compounds and pre-placed fortifications. The Alamo should have a commanding view of its surroundings, and should have few entryways to be defended. Players occupying high-ground positions in the Alamo are tasked with gunning down approaching targets, while others guard choke-points to kill anyone who gets through. A properly-defended Alamo should only be possible to capture with masses of enemies, by slowly whittling down the defenders. Blowing up the building(s) or Breaching the perimeter at an unexpected point are another possible way to overcome the defenses.
A few missions have ended with the players successfully escaping the Alamo and being Extracted. In other cases, dying in defense of the Alamo is itself the intended climax of the mission.
Alamos have been used in both Cooperative and Adversarial missions over the years.