Base Defence

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For the XCOM: Enemy Unknown (2012) version, see Base Defense (EU2012)


An Alien vessel has landed nearby. Our base is in severe danger. As per standard procedure all non-combat personnel and X-COM craft have been evacuated. Alien units will enter the base via hangar doors or the access lift. Defend the base and its vital installations at all costs - this is a fight to the death.

"this is a fight to the death" - implies that when you hit Abandon Mission, X-COM forces decide it is better to die and take the enemy with them, rather than admit defeat, and blow the base up.


An inbound Battleship on a "Retaliation" mission means there is a high chance that a Base Defense mission is in the offing for you. These ships will come in at a very low altitude and at their top speed. They obviously do not care about the well being of the craft, to run it that hard in a full atmosphere!

You can bring down these battleships with sufficient Base Defence Systems, but the game is programmed to just send another on the exact same mission. (To be precise, the game does not check/remove the "X-COM Base detected, send in the battleship" flag). It has been proven that about 6 or 7 Fusion Ball Defences with Grav Shield (total defensive strength 6000) can hold off incoming battleships (hull strength 3000) indefinitely, but since you get no bonus for blowing a battleship out of the sky, not even to monthly score, it is better to let them land eventually and just run the mission. Once you've run the mission, win or lose, the attacks stop coming. The aliens are content that they've done sufficient damage.

Blowing the battleship out of the sky with Aircraft Interception ALSO seems to uncheck the Battleship assault flag, so keep that in mind.

Only battleships actually assault your base. Don't worry too much about other alien craft on "Retaliation" missions, they're just scouts. Shooting these down far away from your base may confuse the aliens as to where your base actually is. Shooting them down will also ruin their base finding mission, saving your base until the next scout is sent to replace the first.

If you decide to dismantle the base while there is a battleship enroute to attack it, the battle ship will head for that base's ex-location anyhow. It will then realise it's been duped, BUT, after consulting its alien masters, will simply head to the next closest base. In a straight line, top speed. HOWEVER, once it arrives there, it will realise that they only have the lift access codes to your old base, and will then simply hang there hovering over your base for ages and ages. In fact, this is due to the nature of the code which sends out the scouts. If successful, it sets the "aliens know where base is" flag, and applies it to the next battleship launched on a retaliation mission. This flag is only removed after a battleship actually manages to land and send troops inside your base. This flag is not removed by base defence blowing up battleships, craft blowing up battleships (I think), or, as I just mentioned, by removing that base entirely. Nor is this flag restricted to that base in particular.

Battle Tips

A good Base Layout Strategy is of paramount importance here. Make sure your base is designed to make it easy to defend.

You really want everybody at home for a base defense mission. If you're caught with your Skyranger off at some UFO crash site, you'll have to defend the base with a rookie, two guys on crutches that were in med-bay, and the antique Tank/Cannon you hadn't got around to selling yet. And in Collector's Edition, you don't get to use the injured guys in med-bay. Even worse, if you don't have a single soldier who is fit for duty, you lose the base automatically. Yes, even if you have 10 Ace soldiers who only have 1 day of recovery remaining, 10 Hovertank Launchers and 5 Hovertank Plasma, and you managed to *almost* destroy the incoming battleship with your 6000 defence strength... you still automatically lose.

Losing or abandoning the battle means losing the base, and everything in it. Although sometimes you can still come out with a positive score for your trouble. Any aircraft which are assigned to this base will be lost... even if they are outside the base at the time. Your Interceptor enroute for shootdown will abruptly self destruct. Your Skyranger with your best soldiers returning from a successful Alien Base Assault will crash and burn.

In battle, the odds are in your favor. You know the layout, it's fully lit (even the Hangars give 20 tiles of vision, despite visual effects to the contrary), and you (should have) designed in choke points.

Equipment assigned to your craft is available for base defense missions, unless the craft is out of the base. Equipment loaded on craft in the base becomes part of the general 80-item pool. If it were otherwise, your psionic troopers would likely be useless for want of Psi-amps.

Battleships will unload both regular aliens and their associated Terror Units. You won't ever see the battleship, but you will get to see all those nifty modules you've been constructing in the Base Planner view, close-up! Base modules have two levels. Soldiers will normally spawn/start in Living Quarters and Storage modules. Aliens spawn/start in Hangars and the Access Lift.

A soldier with a Motion Scanner in an upstairs closet near the access lift combined with a rocket tank at standoff range will keep the aliens at bay until the rockets run out. -see also [Stewart's] strategy on forums.

Blaster Bombs can clear your base in just a few turns, if you launch a few into the hangars and access lift. The problem is getting the program to supply your soldiers with blaster bombs before hitting the 80-item limit.

Hovertank/Launchers are almost as good at clearing out your base, except for the fact that their missiles are more prone to inaccuracy and often veer off and blow up against the wrong walls. Since the game has a "free ammo for HWP in base defence" bug, I don't see HOW you can lose as long as you keep 3-4 of these things in every base.

Doing too much damage to a room can also collapse it, and you will lose that room permanently. Any rooms that can only be accessed by going through the collapsed room will also be lost. Spawn-point rooms such as Living Quarters, Stores, Access Lift, and Hangars (plus the Alien Containment chamber) can't be collapsed.

You can recover lots of alien weapons from a successful base defense. In at least one version of the game, there is a bug where you can collect an extraordinary amount of Elerium, enough to start selling it.

It might be best to disarm all your psi-weak soldiers at this base if there are Ethereals or Sectoids attacking.

If you built a General Stores near the Access Lift, the double doors are a great place for ambushing aliens as they leave the Access Lift. Each chamber will hold two soldiers. Eight soldiers stepping out and firing one or two bursts of laser or plasma shots will almost guarantee that no alien penetrates further into your base. Also, so long as no soldiers or tanks are visible during the aliens' turn, they will not make psi attacks (visible to any alien, not just the psionic alien). The doors have a damage rating of 75, which means they will withstand an alien grenade detonated right beside them. This tactic has one downside against Sectoids: Cyberdisks will explode when killed at the chokepoint and destroy a large proportion of the loot.

Alien Weaponry that has been researched can be used for Base Defense even if it's ammunition has NOT been researched. In other Missions you simply cannot load the ammo in your transport, but when defending a base this is not necessary. The clips will be labeled as alien artifacts, but can be used as if research was finished (except that it cannot be thrown).

[A somewhat unorthodox strategy, forced by necessity, paid off for Zombie: "I armed my best soldiers with the stun rods, hid them in the General Stores next to the Access Lift, and went to work. Every round a Snakeman would appear from the Access Lift, and every round one of my soldiers would stun the alien while another veteran grabbed the body and hauled the poor alien upstairs for observation. I was laughing so hard I almost peed my pants!" - as copied from strategycore. Mental picture of a Snakeman with a pillowcase over his head, zipcuffed arms behind his back, old army sock stuffed in the mouth, being heaved into a darkened storeroom on top of his buddies. "We get $20K for each, alive or dead, right?" --JellyfishGreen 08:34, 14 Oct 2005 (PDT) ]

To Surrender or Not To Surrender

There are only two costs incurred when you lose a base:

  1. You lose the base and everything in it;
  2. You lose points for the soldiers and tanks that died defending the base.

The loss of the base is presumably erased from the annals of history. It's all kept very hush-hush from the public. The non-combat staff and all ships that were at the base are disbanded and are not lost in the battle. X-COM ships vanish without a trace, probably stolen by wretched engineers. The deaths of the soldiers on the other hand are a lot harder to explain, and friends and families are bound to make inquiries.

Therefore, if you find yourself in a situation where you are defending a base against an enemy that you have no hope of winning against, should you surrender without a fight, or fight it out? You will need to base this on who you are fighting with and what you have to fight them with.

For example, a fresh batch of recruits in jumpsuits armed with regulation standard pistols and rifles against an elite squad of Ethereals and Sectopods. This can be classed as a hopeless battle should you not be able to utilise the weapons used by the attackers. In a situation like this, you should cut your losses and surrender without a fight. This will incur a 0 point penalty along with the loss of the base.

On the other hand, should it be against Floaters, you could push on and try your luck.

Alternately, you could also try to kill as many raiders as you can before surrendering. As long as you did not lose any soldiers in the process, you will get a positive score for losing the base. If this is an option, go for it. If not, don't.

It is understandable that some commanders may be too proud to go down without a fight. There is nothing wrong with that, but do try to consider the consequences of any actions before taking them.

Note: All of the above relates to raids on bases when you have more than one base. If you are put into a situation where you cannot save your very last base, then you should consider restarting the game.

If you have have advance warning about a base raid that you're going to surrender-- i.e., a hyperwave decoder, or you can see that battleship making a beeline for your base on a normal radar-- take the time to transfer as much as you can out of that base to other bases before the raid hits. If you've got free hangars in other bases, transfer the craft, save any elerium there, and transfer out all the personnel you can. An obvious thing, but can be quickly forgotten in the shadow of a battleship. --Papa Legba 17:36, 22 Nov 2005 (PST)

You should always transfer pretty much EVERYTHING out of a base even if you expect to win easily. In base defense missions you WILL hit the 80 item cap. You need to thin out your extra gear before the aliens get there by selling the stuff you don't really need and transferring the stuff you want to keep. You don't want to defend with just smoke grenades and electro-flares. My tiny radar bases always have a general stores for that purpose.--Brunpal 01:06, 11 August 2008 (PDT)

As stated earlier, the aliens can attack your base pretty early (especially on superhuman; For example, I got my first raid before laser pistol research was done!)There can sometimes even be Base Defense mission on Veteran on the first month (I got mine on January 8th against Sectoids and i had to quit because they had psi abilties and all I had were rifles and pistols with only 4 people with laser rifles but everyone was going berserk) - pistols and grenades may work well against sectoids, but are pretty worthless against cyberdiscs - still, even in this case, the answer should be clearly "not to surrender!" - In the early stage of the game, there are two ways of winning against cyberdiscs: first: get a soldier to stand near it and provoke a reaction shot by a different alien and hope it hits the disc, leads to its death and doesnt let it explode. Or just stun it - 2-3 soldiers with stun rods do a great job there, especially in a base with its plenty of cover.. User: Player11 23:47, 23 oct 2006

"If you're going to play a base defence in the early months (I had mine on January 12th on Begginer Mode) make sure to stay out of sight in corridors to ambush any alien stragglers. This tactic will be especially useful against any psionically capable aliens (Sectoids and Ethreals) also it would be useful to throw proximity grenades next to the access lift doors. These tactics ensured the survival of XCOM in the first month of the invasion. It's also recommended that commanders level the acces lift so soldiers cannot be blasted apart as soon as they discover an ambush behind the lift doors. Even though you may severely damage the access lift you won't lose it like the other facilities. - User:WakkaDakka 14:03, 19th May 2015

Alien Deployment

Rank Beg./Exp. Vet./Gen. Super.
Medics* ** ***112

*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 Medics, Leaders and Commanders.

Destruction Of Base Facilities

Excessive damage to the structure of your base can result in some modules being completely destroyed, even if you win the overall battle.

Certain modules have "special" tiles in them, and if all of these are visibly damaged (or completely obliterated), then that module will be considered lost. Furthermore, if any other modules used the destroyed areas for passage to the access lift, they'll be forfeit as well!

Fortunately, these tiles only exist on the upper floors of your base, and the aliens make no attempt to target them once they encounter your troops (... if they don't encounter your troops, they'll spend the first few dozen turns beelining these tiles - and if they're allowed to reach one under the CE version of the game, when they fire it'll crash). While explosions on the lower floors may damage the upper flooring, they leave the furnishings unscathed - even if the floor gives way completely, they'll still hang safely in midair. Presumably the builders had the foresight to attach them to the ceiling with wires.

For example, the Fusion Ball Defences contain eight gun arrays made up of four tiles each. All 32 of these must be destroyed in order to take out the module.

Grav ShieldTargets-gravshield.gifSmall Radar SystemTargets-smallradar.gif
Hyper-wave DecoderTargets-hyperwave.gifWorkshopTargets-workshop.gif
LaboratoryTargets-laboratory.gifMissile DefencesTargets-missile.gif
Large Radar SystemTargets-largeradar.gifLaser DefencesTargets-laser.gif
Mind ShieldTargets-mindshield.gifPlasma DefencesTargets-plasma.gif
Psionic LaboratoryTargets-psilab.gifFusion Ball DefencesTargets-fusion.gif

The remaining modules - the Access Lift, Alien Containment, General Stores, Hangar and Living Quarters - are completely indestructible, and can take any amount of damage without danger of collapse.

Note that squares that have been emptied due to battle damage fall victim to the "paying for dirt" bug. See Known Bugs for more info.


Base defenses can be DEADLY.

But not for the reason you think.

  • When you have more than 80 items present at a base, the game will only make available the first 80 items on the stores list (See '80-item Limit' in Known Bugs.) . This will result in conventional weapons/stun rods/electroflares/etc. being available instead of lasers/plasmas. To prevent this there are 3 possible solutions before that Battleship strikes the base:

1) Transfer the unwanted items to another base.
2) Load your transports (if there's any) with the unwanted items and launch them before the UFO lands. This saves the work of having to transfer those items back but if the base gets destroyed the transport will disappear and the waypoint to which it was headed will remain visible for the rest of the game.
3) Sell unnecessary stuff. "Imminent Alien Invasion Yard Sale! Everything Must Go!"

  • If you had 40 heavy plasmas in your base and forgot to get rid of those 40 smoke grenades, all your troopers would be lacking the clips for the heavy plasma. Good luck defending a base with nothing but 40 smoke grenades. [Darksun: Lucky for my stupid self, I had a couple of laser tanks to convince the aliens to lend me some ammo.]
  • A relatively easy way to counter this problem is to keep your stores of "conventional" weapons low and keep 20 or more Laser Rifles in base inventories. Laser Rifles are high on the inventory list and are distributed before most other items. As they do not require ammo, any laser is immediately combat-ready.
  • Another ANNOYING bug I find is the disappearing bodies and booty from aliens. This is related to the 80 item bug problem.
  • Also much feared is the dreaded "walled off section" or Base Disjoint Bug. Sections of your base along the extreme south and east edges will often be separated from each other by dirt walls. Heavy plasma and blaster launchers are needed in this scenario to excavate the dirt. Soldiers spawning in Stores or Living Quarters here may be trapped for the duration.
  • A much nicer bug is that HWP's automatically get a free load of ammo (which is added to your stores if unused). Note you still need at least one soldier as aliens landing at an unguarded base - or a base defended only by HWP's - take it without a fight.
  • If you have a combination of many soldiers and few spawn points, soldiers will start taking alien spawn points. Which usually means less aliens (unless you built a lot of hangars).
  • The last point also applies to aliens: if the aliens don't have enough spawn points they will start using the X-COM ones, which can turn base defense into a nightmare!
    • There are 8 alien spawn points in the Access Lift, and 15 in each Hangar. As such, you need at least one Hangar to avoid aliens spawning in unexpected locations. On Superhuman difficulty, a second Hangar is required for safety, as there are 22-28 aliens and a single Hangar only gives 23 alien spawn points total.
  • On a related note, X-COM can only have a maximum of 40 soldiers on base defence. HWPs are prioritized and will be spawned before any soldiers. Each HWP counts as 4 soldiers. Therefore a base with 9 HWPs will only spawn 4 soldiers. Having 10 HWPs in base might cause your game to crash. (it will attempt to create the Equip Soldier screen but will face problems).
  • Elerium collection from a base defence mission can only happen if elerium pods are generated in the battlescape. If you have 10 units in storage, the game will generate 10 pods in the base, given sufficient space in the object table. But because the way the game automatically converts 1 pod into 50 units at the end of the mission, you're basically getting 5000% more elerium from your own pods (those that don't get destroyed, anyway). None of them come from the aliens! This will only happen in earlier versions of the game.

See Also

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