Base Layout Strategy
Your base will, at some point, be raided by angry aliens looking for revenge for all those UFO's you've taken out. Unlike most places you'll have to fight, you have control over the layout of your base, so design it wisely.
The key point for base defensibility is to minimize the number of points of entry. Aliens usually come down into your base through hangars and the access lift (although they can emerge on other locations - see the last comment on Base_Defense#Bugs). If you group your hangars with the access lift, then build only one module connecting to this part of your base, that module becomes a choke point for base defense. A long chain of rooms becomes a long choke point ripe for mining with Proximity Grenades and other dirty tricks. This way, you can line up all your units against one point of entry instead of having to spread them out to cover four or five.
The access lift is always the first unit built, so it becomes the obvious candidate for this single point of entry. Place it on the grid so you leave enough room to build hangars on one side of the lift, and everything else on the other side. I'd recommend building a General Stores on the lift opposite the hangar. A general stores is needed for any base, and it gets completed quickly. Once the general stores are completed, build all new facilities (except hangars, of course) off of that.
Overhauling the Starter Base
At higher difficulty levels, alien retaliation can often come MUCH sooner than you might be ready for-- before you have base defenses built, and before you have powerful weapons to arm your troops with. Considering that the access lift and all hangars serve as entry points for the invaders, the starter base has all the defensive virtues of a screen door:
The hangars and lift are distributed as widely as possible through the base, giving the aliens a wealth of breach points and enabling them to assault any defenders from all sides.
With this in mind, I recommend a partial rebuild of your initial base ASAP. Build two hangars next to the topmost hangar to replace the lower two, and a living quarters down between the two lower existing hangars to replace the one next to the access lift. Place any other new modules on the lower half of the screen, where they will not touch the lift or the new hangars.
As the new living quarters and hangar are completed, dismantle the old ones. It should now look something like:
Of course, place any other new facilities with a mind to preserving the new choke-point at the grav lift (i.e., nothing adjacent to the top hangars.) This layout isn't as fortified as many new-build bases can be, but it's a good compromise between defendability (one choke-point at the lift), available space, expedience (it converts from the standard layout in minimal time), and, especially, cost. It only costs the price of two hangars and a living quarters ($800,000) to overhaul your base this way-- a cost that can be covered by the proceeds from, say, one small landed UFO. Dismantling the existing modules however accrue base maintenance costs of $720,000 per month due to the Paying For Dirt bug.
The previous example was a one chokepoint base that is easy to defend, however it is by no means the only way to build the base. Don't be afraid to try different variations of the base to suit the strategy that you intend use.
If you want to attack the aliens from two sides, the simple addition of an extra module can modify the one-chokepoint base into a two-chokepoint base. For example:
The above base is designed to work like the popular defendable base layout, but also provides a second access point into the hangars. Although this means you have to spread your soldiers out to cover both points, so do the aliens.
With this example in particular, the small radar module provides an empty hallway. If you like to use rockets, this one access point gives you a clear shot into the hangars. A few well placed rockets can cut down the opposition considerably.
If you really like to use rockets but can only handle aliens entering the map through one entry point, consider the following variation.
It may take a while to re-order the base in this manner from the starting setup, but the end result provides you with a clear firing corridor into the hangars with plenty of cubbyholes to hide in. All of the aliens will have to funnel through the small radar module, which is an open space with no place for the aliens to hide.
One disadvantage of this base however is that if the workshop suffers too much structural damage, all facilities past the small radar will be destroyed as they'll be disconnected from the access lift.
No matter what layout you eventually use, remember that the more complicated it is the longer it may take to construct and the more money it will cost. The last example requires moving a module that was already present in the default base. i.e. the general stores.
Secondary Base Design
With your second and subsequent bases, you can build it securely right from the beginning. The most secure design uses the Access Lift as a choke point between the hangars and the rest of the base, as pictured above, but puts it to the side, out of the way, and also adds a long corridor between the Access Lift and the rest of the base:
The exact facilities built are up to the individual player, although a hyper-wave decoder, stores, and living quarters are generally the most important facilities to build first. Even if you don't plan on stationing any X-Com craft at the base, you should also build at least one hangar (where aliens will spawn preferentially during a Base Defense mission) to prevent them from spawning deeper inside the base.
Be sure not to put the Access Lift on the southern or eastern borders of the map to avoid the base disjoint bug.
If three hangars are not needed, additional base facilities can be added, as long as they are not placed adjacent to the hangars:
The design with only one hangar, however, runs the risk of having aliens spawn in areas other than the hangars and access lift, as one hangar plus one access lift only provide (15 + 8 = 23) alien spawn points, and a Battleship on Superhuman difficulty can carry between 22 and 28 aliens. Especially troubling is the fact that Terror Units have the lowest alien spawn priority and thus are most likely to show up in your base. This might result in some particularly nasty units spawning behind your lines, quickly turning an otherwise easy Base Defense into a nightmare!
One drawback of the above design is that it takes 52 days before all the facilities in the "choke line" are completed and the base is fully defensible against invasion. The following variant moves the Hyper-Wave Decoder to the right of the access lift, allowing the decoder, one hangar, and stores plus quarters to all be complete within 26 days.
Moving the hyperwave out of the choke line also makes it less likely to be destroyed by a stray blaster bomb. In the above designs, if the hyperwave gets destroyed, all modules beyond it will be destroyed as well. (Living quarters and stores are immune to destruction.)
Time-Efficient Corridor Build
The following designs are an attempt to greatly speed up build times without sacrificing much (if any) defensibility. The base on the left is an example of how to start a new base if you don't yet have a Hyperwave Decoder. This would get you 2 Hangars, a General Stores, Radar, and Living Quarters by day 26. You can of course skip the Large Radar if you have already researched Hyperwave (in which case you would no doubt place the Hyperwave above your first Stores).
The base on the right is a complete version of this base. Notice that the General Stores above the Access Lift is the only facility that has been removed if you did not build the Radar, so if you are paying for dirt, you will not suffer greatly. This build has only 5 wasted spaces, which is less than any other defensible build using 3 Hangars. If you really want the extra corridor space, you can skip the bottom-most Living Quarters. The corridor is, of course, indestructible, as it is done with General Stores. As a warning, if you have not eliminated the base disjoint bug, you will probably have to rotate the base 90/180 degrees ,or you will find your soldiers sealed off from the aliens. Furthermore, if you expect to be attacked within a short period of time, you may not yet have swapped your choke point from the Access Lift to the General Stores on the left.
Lastly, the base on the right is an alternative 2 Hangar build.
If you have a program to overhaul your starter base, the base on the bottom left is what I recommend. The picture on the right shows a good choice for immediate additions to it. You would of course want to add another Living Quarters if finances permit (don't forget to staff your Laboratory or Workshops).
Corner ambush build
I decided to put a little variety into my base defences. I like this pattern because i get to ambush the aliens as they turn the corner of the first general stores. Let THEM see how it feels getting whacked at blind corners! Also, notice how that fights will only occur in or near indestructible modules. Too many exploding cyberdisks convinced me to do this.
The fact that the corridor is 5 units long is SIGNIFICANT. (the prior designs are actually only 3 modules long, because aliens spawn in the access lift, which has also has doors which block LOS and fired shots) Because alien sight is 20 squares. 5 modules is 45 tiles.This means that I can in fact keep 6 units (3 standing, 3 kneeling) at the north end of the North East corner module. I place scouts/spotters with a motion detectors in the rooms and round the corners of the corridor of general stores. The spotters pop out diagonally for mutual surprise, then hide again. Then the 6 snipers at the end of the corridor shoot down the corridor killing enemies in it. 45 tile distance means that no alien can turn the corner and walk far enough down the corridor to spot and shoot at my snipers in 1 turn.
Missile Defence Module Build
This build takes advantage of the missile defences unique structure to force aliens to enter from the right, take a big detour, then exit to the north. This will likely severely hamper enemy blaster bomb efforts. However, by the same token, this is very likely to wind up causing your missile defences module to get blown up by blaster bombs, which will disconnect the rest of your base from your elevator, and make you lose most of your base. Be warned.
Floor Plan Comparison
Here's a comparison of the ground floor plans for X-COM base modules, in case it helps you design your base. For images, follow the links or see X-COM Base Module Images:
- 3-wide "central plus sign" corridors:
- Central room:
- Unique modules:
Most modules have a fairly open second story, with the exception of Missile Defences, General Stores, Alien Containment, and the Psionic Laboratory. Still, half of the rest have a fair amount of clutter that might hide aliens.
- Corridors attaching to the right topside of a hangar (on the grid, the east side of the north wall) corner come through a ground-level hangar door. Corridors attaching anywhere else don't get a door. See picture at X-Com_Base_Terrain.
- The ideal case is to have your choke point aligned vertically on the base grid, lined with general stores. The general stores have large, doublewide doors that open out onto the vertical corridor. These doors will give your your troops and tanks cover, and let them pop out to make attacks at will.
- It is also desirable to have a base facility containing a door directly facing the Access Lift (such as the Hyper-wave Decoder). A scout can pop out at the beginning of a turn and spot units coming down the corridor without risk of being shot due to the "mutual surprise" rule. Other units with indirect-fire weapons (grenades and blaster launchers) can then safely attack the spotted alien(s). Direct-fire weapons can also be safely used if a thick-enough smoke cover is present.
- Troops will spawn primarily in the Living Quarters and General Stores . Tanks will spawn mostly in the General Stores. You might want to cluster these units around your choke-point so your units will be where you need them.
- When constructing a base, be very careful that you avoid the Base Disjoint Bug where modules along the right and lower edges of the map can get sealed off. Improper placement will often end up forcing you to cut through the very tough walls and dirt blocks with heavy plasmas or tank lasers. However, with proper placement of modules, the sealed off walls can be very advantageous in controlling the flow of aliens. XcomUtil users cannot take advantage of this as the setup tool for this utility automatically rips out the walls.
- You can (and should) keep the poorly positioned rooms of your starter base up until the battleship is closing in. You can dismantle rooms instantly as long as they are unused. Since maintenance costs are buggy and listed values are always wrong, you are always better off keeping a room even if it is unused... at least up until the point the aliens arrive or you need the room.