Base Defence Systems
Aliens will soon start finding - and attacking - your bases. At your disposal are a number of measures you can use to prevent these attacks from taking place.
Three fusion ball defenses with one grav shield is usually enough to prevent any UFO from landing, while six or seven FBD's with a grav shield pretty much make the prevention a sure thing.
It could be argued that it is better to simply allow the aliens to land and fight it out using your soldiers. Once aliens have found your base, they will continue to attack it until one of their craft make it past your defensive measures, and the crew are beaten inside the base. Once this happens, they will have to locate the base again before resuming attacks.
Given that the base defense screen is boring and gets repedative quickly, many players opt to not pay maintanance on defensive measures and simply allow the aliens to land. Given the advantages you have when fighting in your own base (where you have a very good idea of where the alien units will begin and what the layout is), and the fact that an alien commander is always present for you to capture, the defensive systems could be viewed as redundant.
[ NKF: With the exception of the mind shield, which helps at all times because it reduces the amount of untimely base visits from your friendly local alien invaders. Also, just to put up a very minor counter argument for the other modules: An impenetrable base defence system does have the advantage of giving you a bit more time to prepare for the attack if you think you're not ready to handle it. Say you're rearranging your base, or you've got a few fusion tanks still on the assembly line. When you're ready, it's just a matter of removing the grav-shield to halve your defensive power, and let the raiders through to finish them once and far all. Of course, it's still repetitive as anything, and Grav Shields are expensive to rebuild over and over for each attack! ]
Offensive Defense Faculities