In the beginning, a base starts with very little. You have a small radar, a living quarters, a laboratory, a workshop, a general stores, and three hangers. With the money you start with, you should build an alien containment, a missile defence, a general stores, a large radar, and a living quarters. The starting general stores will fill up very fast. Capturing aliens is the only way to win the game. Missile defenses protect you. A large radar gives you a larger detection radius. Finally, without a second living quarters, you can only have 32 scientists.

Other facilites require researching to be able to build. More advanced defences than missile need to be researched before they can be built. The mind shield, hyper-wave decoder, psionic laboratory, and grav shield are other add-ons that need researching. These give you advantages over the aliens.

The most important are the grav shield and the hyper-wave decoder. The grav shield works when the aliens try to destroy an X-COM base. The grav shield pushes the UFO back after the defences have fired, and allows those defences to fire again. This doubles the effectiveness of the defence of your base. The hyper-wave decoder is probably even more important. If a UFO is detected by a radar station in a base that has a decoder, a screen will appear that tells about the UFO. The decoder knows where the UFO is going, what it plans to do, and what it has aboard. This can help stop terror sites before they start.

Once the aliens find out where an X-COM base is, they will send UFOs to destroy it constantly. If a UFO is still flying after the defences have fired, a base defence mission occurs. This makes defences the most important base add-on. Without defences, a base can be destroyed without a fight. The aliens have a big advantage when they attack your base, you cannot abort. If you lose a base defence mission, the base is destroyed.