Your base is equipped with high-tech defence systems designed to stop a particular sort of attack. It might have anti-aircraft missile launchers, or be surrounded by anti-tank landmines. It could even be equipped with lasers to shoot down spacecraft, or teleport jammers to stop Sontaran commandos from blipping into the middle of your HQ.
Pick one form of attack. The base provides a +8 bonus to Offence or Defence against that form of attack (or reduces enemy Mobility by -8 if they are using a particular form of transport). You can choose to split this bonus to give a smaller bonus to both Offence and Defence if you want, as long as it adds up to +8.
For example, a base equipped with anti-aircraft missiles might also be hardened against attack from above, giving it a +4 bonus to Offence and Defence against air attack.
You can take this Trait more than once; each time it applies to a different sort of attack.