Alien Base Assault

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The aliens can build a maximum of 26 bases around the globe: 1 for each zone that has a land mass (Alien Base Mission) and then 1 for each country (Infiltration Mission), with the exception of Siberia. There could potentially be less: Only fifty (50) Geoscape "tokens" (bases, waypoints, ships, etc) can exist on the globe at any given time, and because you have to share this space with pretty much everything that you can interact with on the Geoscape, a base can only be created if there are unused slots, at the time the mission is supposed to be completed.

Note: UFO flight pattern investigation has found that the base is placed as soon as the battleship arrives in the atmosphere, so if it shows, the aliens have a base.

Locating an alien base

Alien bases do not show up on the Geoscape by default, they are hidden until you find them. To locate one, an X-COM craft must patrol in its vicinity for a short time. Also, X-COM agents may report a base location to you at the beginning of the month, but that shouldn't be relied upon.

There are several clues that indicate the presence of an alien base:

  • A large UFO performing an Alien Base mission may indicate the creation of a base, or a preliminary scouting mission. Several UFOS running alien base missions simultaneously indicate the construction of a base, and the base may be built even if you interdict all of those UFOs. See: Alien Missions
  • An Alien Supply mission will always land directly on top of a base; patrol the location until you find it.
  • For each nation that the aliens enter a pact with, they will also build a base somewhere in that nation's region-- but not necessarily within that nation's borders.
  • Check your graphs for UFO activity by region. A spike of UFO activity in a given region probably indicates the construction of a base there.
  • Alien bases are numbered by generation, not by detection. If you already have destroyed 3 alien bases before discovering "alien base 5" you should continue your search to find number 4.


There are two ways of destroying a base:

  • Destroying the contents of the room on the second floor of the Command Center renders the base destroyed for all intents of a base assault. Specifically, you must destroy all 16 sections of the 4 purple octagonal tables (normally these are UFO Navigations). A single Blaster Bomb to the upper level will usually set off a chain reaction: if you hear 16 explosions in a row, you're all set. If you abort the mission after destroying the Command Center, the mission will end and the base will be destroyed. Be sure to move all your troops onto the green tiles in the "exit areas" on the upper level. Any troops not on a green tile (including those on the red lift tiles in the exit rooms) will be counted as MIA.

It is often much easier to infiltrate the Command Center by shooting through its walls (and ceiling) than by entering it via its doors and access lift. See Destroying Terrain#UFO walls for more details.

  • Kill all the aliens. This method will of course give greater rewards, but is sometimes more difficult to achieve, especially in the case of Chryssalids, Sectopods and Cyberdiscs hiding in obscure distant closets, waiting to ambush your troops as you search for the last remaining stragglers.

Recoverable components

The components that can be recovered from an alien base varies, dependent on what modules are used in its layout (see Alien Base Terrain). Most of the recovery value from an alien base assault will come from recovered alien armaments. See UFO Recovery Values#Alien Base Assaults for more details.

Alien deployment

Rank Beg./Exp. Vet./Gen. Super.
Medics* **111

*Ethereals use Ethereal Leaders in place of Navigators, Medics, and Engineers.
**Snakemen use Snakeman Soldiers in place of Medics.
***Mutons use Muton Soldiers in place of Leaders and Commanders.

Smash and grab

Alien bases can be a very quick and dirty (dangerous) way to get Elerium, using what we call the "smash and grab" mission. Raid them by seeking out the storage room with power units. Destroy the power units without using explosives, and pick up the purple Elerium canisters that were underneath. Boogie back to the entry lift, and abort the mission. You get the Elerium and the base is not destroyed. This means you can raid as often as you dare. Consider grabbing a few pieces of alien equipment on your retreat to replenish any spent equipment during the raid, and perhaps if you are lucky enough to knock out a high ranking alien, drag it back to the entrance to have it abducted.

Note: Raiding the supply Supply ships for a base rather then the base itself may be much more profitable. A base populated by docile aliens (floaters) in a convenient terrain (flat ice, flat desert) will have an easy to grab supply ships. The constant 150 - 900 elerium/month are well worth the 150 points penalty for letting the base exist.

If you want to do repeated Smash And Grabs on the same base, you must not destroy the base Control Center, and you must leave at least one alien alive in the base.

One tip is to knock out an alien, bring it somewhere safe where you can watch over it (such as the small storage rooms with the small grav-lift), then revive it with a Medi-Kit. Proceed to wipe out the rest of the aliens, but do not damage the command consoles in the command centre. As soon as the base is neutralized, initiate a recovery mission and gather as much equipment (and corpses, if you're really stretched for cash) in the green entrance pads before aborting the mission. Though this may seem time consuming, it does allow you to reap the rewards of a normal successful mission.

The mission will be counted as a technical failure, but that's nothing to worry about, as the activity points, loot and experience will tell you otherwise.

Many (or most?) alien bases will not have power units, or any Elerium. Recent research has shown that attacking a base's Supply Ships is likely to be more profitable (and less dangerous) than attacking the base itself, and a more reliable source of Elerium. See UFO Recovery Values#Alien Base Assaults for more details.



Camping exploits two quirks of the game: aliens cannot fire conventional weapons up or down through lift tiles, and will fall prey to the "Vertical Waypoint Bug". When the aliens attempt to fire Blaster Bombs up the lift shaft, this bug will cause the bomb to travel south, not up, keeping your troops safe on the upper level. The vertical waypont bug only exists in the "Collector's Edition" (Windows) version of X-COM; the camping strategy cannot be safely used in any other version of the game.

Note that when your starting point is in the far south of the map this is not entirely safe. The aliens have about 10-20 blaster bombs. Those are able to destroy even the earth terrain behind the base walls made of alien alloys. After two shots the alloys at the south wall of the lower level of the starting point are destroyed and the aliens begin to "dig" a new hallway. If this hallway reaches the edge of the map the blaster bombs following will fly off the map and then reappear on their next waypoint, which is located on the unit blocking the north west tile of the elevator. The result is obviously the death of almost all of your units in that area and the destruction of any items located there. If your other starting point is for some reason not suitable for camping (for example because no aliens are passing it or being located in the far south, too) abort the mission and try again.

To use the camping strategy, have all your troops retreat to the upper level of the access lifts (the green-tiled rooms). Position a soldier on each square of the lift on the upper level so aliens cannot use the lift to come up (this requires a minimum of eight soldiers or two HWPs). The simplest thing to do is then wait until turn 20+, when all the aliens will exit their own encampments and mass on the lift squares below you. You can then shoot them. To reduce the chance of your troops accidentally shooting each other, have them only shoot at aliens located directly beneath them.

Camping tactics can also be used on other lifts found within an alien base, with varying degrees of effectiveness. Rooms which have only one entrance (including the control room) are best, as you do not have to guard your back. However, these rooms are dangerous to approach and clear of aliens in the first place.

Camping cannot be used in Sectoid or Ethereal bases unless your troops have extremely high Psionic Strength and are able to completely withstand Psionic attacks (especially Mind Control).

Camp and scout

Instead of waiting, you can also choose to send a scout down the lift every turn. Have the scout turn in place to survey the immediate area; if any aliens are spotted, you can send other troops down to shoot or grenade them. Depending on how many TUs the alien(s) have left, your scout may get fired upon when descending the lift, or the other soldiers when firing upon the aliens. Dropping a Smoke Grenade in the center of the lower lift room can reduce the number of aliens that can see your troops while they scout.

If the coast is clear, you may wish to have soldiers retrieve nearby corpses or alien gear, so as to prevent them from being destroyed if a Blaster Bomb is set off in the room. You may come under fire from aliens that were not initially visible, however, and it is easy to miscalculate how many TUs are needed to retrieve items and return to the upper room.

Camp and grab

After killing several aliens, most of the surviving aliens will panic. If you wish to use camper tactics during a "Smash and Grab" mission, it is easy to approach one of these survivors, stun him, drag him up the lift and use a Medi-Kit to revive him (don't use a Terror Unit, as they have in-built weapons). You can then kill any remaining aliens you see and collect Elerium and whatever other goodies you wish.

Base Farming

Main article: Base Farming

Base Farming involves finding an Alien Base and then constructing a base on top of it. Once you have it set up with a radar, stores, men and a Skyranger, you may begin farming off the base's supply drop offs. Between 150 to 900 Elerium is worth the -150 points you'll get at the end of the month. And the best part is if you get bored of it then you just attack the base and then dismantle yours so it can be built over another one, and you may have up to seven of these if you do not have Research/Manufacturing/intercept/Radar bases.

Mind control

Once you have a large number of psi-talented troops, it is easy to win all combats with ease: Mind Control one alien, have it seek out others, then Mind Control any aliens it spots, and have them explore further. Disarm them, then line them up for slaughter.


  • Somebody turned the lights out. Alien bases are dim, but your soldiers can still see pretty far without electroflares.
  • Remember those Proximity Grenades; you can't guard every intersection. (Unfortunately, a Chryssalid will almost unfailingly trip it, shrug it off, and run up to perform its zombifying ways on your troops.)
  • The alien base will be staffed by the same race of aliens that ran the "alien base" or "alien infiltration" UFO mission that set the base up, and the aliens that run the "alien supply" missions to that base. See Alien Missions for more detail. They will have their associated terror units there.
  • Alien bases populated with Snakemen deserve special caution. The high mobility of the Chryssalid makes it especially dangerous here -- it can run around three corners and still have enough time to infect your troops.
  • The Alien Base Commander and a couple leaders will usually -- but not always -- be hanging around the Control Center. Expect some of them to have blaster launchers in the mid-to-late missions.
  • Alien Engineers hang out around the Storage Loft sections. Many of them will be armed with blasters as well.
  • Don't linger long in the rooms beneath the access lifts, and stay spread out at all times. There WILL be aliens with blaster bombs here, and they will be happy to use them on any clusters of your troops.
  • If you are trying to capture a Commander, be aware that before entering the inner door of the command center, if the commander and leaders are not killed or incapacitated before the end of the turn where the door is opened, they may fire their blaster launchers and kill themselves accidentally. Or, worse, they might fire their blaster launchers CORRECTLY, and kill off your entire squad.
  • Also be aware that bases staffed by Mutons will never have a Commander; Mutons can only be Soldiers, Engineers, or Navigators. Just like the Battleship, you still have to contend with Engineers and Navigators armed with Blaster Launchers, though.
  • Feel free to throw explosives around to open up the terrain, especially in those garden areas. It's your enemy's base, after all.

See also

Alien Base Terrain

UFO Badge X-COM: Enemy Unknown/UFO Defense
Mission Types:Crashed UFOLanded UFOTerror SiteAlien BaseBase DefenceCydonia