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. 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. Now, the only points of entry the aliens can invade through are the hangars and the access lift. If you cluster those together, and keep them all isolated from the rest of the base, you can turn those areas into a nice little kill zone and keep the aliens contained within.
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.
Make sure that you build nothing else adjacent to the hangars. This way, any aliens invading the base will have to come through the access lift, and so it becomes a nice little bottleneck, where you can mow the aliens down as they come through.
Overhauling the Starter Base
Base layout becomes quite important, as aliens can only come down into your base through hangars and the access lift. If you grouped your hangars with the access lift, then built one module only connecting to the access lift, 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.
Corridors attaching to the right of a hangar come through a ground-level hangar door. Corridors attaching to the left don't get a door.
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 breech 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, 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.
Notes
- The ideal case is to have your choke point aligned vertically on the map, 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.
- 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.
See Also
Base Defense | Base Defense Measures (UFO Defense) | X-Com Base Terrain