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As the invasion progresses, the Aliens will begin to construct hidden underground bases on Earth. During the construction of the base intense UFO activity will be present transporting supplies and personnel to and from the base site. As long as the Alien base remains operational, it will continually increase panic in the region it is in and eventually cause the region to abandon the Xenonauts project, as well as increase the rate at which the Alien Ticker increases.

Another note is that as the invasion progresses, the Alien bases will eventually increase in size, making destroying the bases significantly harder, but increasing the potential rewards.

Alien Bases will show up as a purple marker on the Geoscape, under the name "Alien Outpost", "Alien Base", or "Alien Fortress", depending on the size of the base.

Once the aliens have constructed a base they will send frequent Supply Run missions to it. This does not seem to affect the base for purposes of gameplay, however, the supply UFOs can be used to locate the base, and can also be shot down/assaulted.

It should also be noted that Alien Bases can hold some of the largest Alien forces the player will fight against.

Alien Base Purpose[]

A functional Alien Base provides two benefits to the Aliens.

  1. It constantly lowers the region's financial contributions to the Xenonauts project, if left operational it would eventually force the region to abandon the project.
  2. Further raising the Alien Ticker Rate, meaning the invasion will advance faster than if the base was not established.

Either way, an Alien Base should be removed as soon as possible.

Locating an Alien Base[]

Alien Bases can often be found randomly regardless of their location, but this is rather unreliable. On the other hand, interceptors and dropships will always detect a base the second it passes within their radar range, meaning that sending an interceptor to a location where the player suspects there might be an alien base is often the most effective method of finding a base.

Of course, interceptors have a very small radar range, so sending out an interceptor to look for a base is usually a waste of an interceptor if you don't have a good idea of where there might be a base, especially since that interceptor could be standing by to chase down UFOs. There are a few ways to deduce a base's probable location before you detect it:

  • Identifying a UFO that you think is on a Construction mission. These missions are carried out by Corvettes, Landing Ships, and Carriers (Medium, Medium, and Massive size respectively) in that chronological order. A UFO on a Construction mission will land, stay on the ground for a considerable length of time, and then take off once it has successfully built a base, at which point it will quickly disappear. Once it has taken off, sending an interceptor to the place where it landed will reveal the base. However, you may have instead seen a UFO on a Research mission, as that type of mission can also land and is carried out by similar UFOs. Nonetheless, it is often prudent to check where they were on the ground anyway, just in case it was indeed a Construction mission.
    • UFOs on Research missions that land tends to hang around for a while after they take off, and will sometimes land again. This is a stark contrast from Construction UFOs, which will only ever land once and will quickly escape to space after they take off.
    • Often it is better to attack the construction UFO while it is landed as opposed to allowing it to build a base, considering the drain of monthly funds and speeding up of the ticker that alien bases cause. The exception to this is if you have yet to take down a base in your playthrough as taking one out grants access to several research projects.
  • UFOs on Supply Run missions will follow a similar flight path to Construction missions, except that this mission is only ever carried out by Corvettes and Landing Ships, making it easier to identify. Supply Run missions will always land directly on top of a base; assaulting the UFO while it is landed will reveal the base as your dropship is about to arrive. At this point, you may want to consider attacking the base instead of the Corvette/Landing Ship.
  • If you have the Quantum Cryptology Center, you will see exactly what mission each UFO is on, so this process is made extremely easy.
  • A territory with an Alien Base in it will see its projected funding drop by $1,000 every twelve or so hours. This is easily noticeable if you're looking for it.

In-game Briefing[]

Drop-ship (Drop-ship Name) is approaching (Alien Base designation). Our strike team is preparing to breach the access tunnels and launch a ground assault on the alien installation.

Elimination of all alien units in the installation will allow us to capture the maximum amount of recoverable technology and is the best-case scenario. Alternatively, if we can locate and destroy the base power cores, we will be able to evacuate our troops without clearing the base. We would recover no technology in that instance, but the alien base would still be destroyed.

Expect large numbers of heavily-armed enemies in the base, so be careful. Good luck, commander.


  1. Eliminate all Alien forces


  1. Destroy the Bases Power Generator.
  2. Evacuate all surviving personnel. (Failure here will still result in base destruction)

Base Sizes[]

As a base's size increases, the amount of Aliens on base slightly increases as well as far more deadly ranks of enemies being on base.

  • Small base (alien outpost)
  • Medium Base (alien base)
  • Large Base (alien fortress)

Control Room and Power Generator[]

All Alien Bases house a Control Room and Power Generator Room.

The Control Room is the brain of the base, and the base's commanders (Officers, Leaders and Praetors) all reside there. It will be the most heavily defended area of the base – however, if all the Aliens in this room are removed and a soldier is occupying it, the entire base will be revealed as your men gain access to the base's security cameras. This of course makes it significantly easier to hunt down any remaining aliens.

The Power Generator is the heart of the base, destroying the Power Generator will destroy the base, and ties in with the alternative win condition. If you do not believe you have the manpower to eliminate all of the base's occupants, or you simply have little reason to expend the manpower to take over the base, the destruction of the power generator will result in the base being destroyed, and evacuation being available.

A bases Power Generator is closer to the base entrance than the Control Room. It is thus easier to simply destroy the base than capture it.

Base Complement[]

Alien Outpost Alien Base Alien Fortress
Rank Caesan Sebillian  Andron  Caesan Sebillian Andron  Caesan Sebillian Andron
Non-Combatant 2 2 - 1 - - 1 1 -
Guard - - 2 - - - - - -
Soldier 7 9 8 - - - - - -
Warrior - - - 11 11 11 - - -
Elite - - - - - - 15 12 11
Officer 1 1 1 - - 1 - - 3
Psion 1 - - - - - - - -
Leader - - - 1 1 - 1 3 -
Other 2 2 2 6 6 3 6 5 5
Praetor - - - - - - 1 1 1
Total 13 14 13 19 18 15 24 22 20


  • In the original X-COM, a strategy/exploit was to keep an Alien base operational and to continually attack it, kill most units in the base and steal everything not bolted down. This could then be repeated as many times as the player wished. In order to make this an ineffective tactic, Goldhawk has made it so that Xenonaut troops cannot evacuate until the power core of the Alien Base is destroyed, while destroying the core will destroy the base as well.
    • Another strategy/exploit in the original X-COM was to keep the base alive and instead build a base of your own on top of it, then "farm" the supply UFOs (this could be combined with the above tactic). While this can be performed to an extent in Xenonauts, it is far less viable as an alien base's mere presence on the Geoscape is far more dangerous.