A defense system consists of various weapon emplacements defending the exterior and interior of the installation. A defense system can have any attack effect with a cost no greater than twice the installation power level. Their attack bonus is equal to the power level.
Installations may have defenses beyond just their Toughness and environmental systems. The most common is for some or all of the installation’s Toughness to have the Impervious modifier (at the usual cost of 1 point per rank) making it largely immune to minor damage or attacks. This may come from hardened or super-science materials or special defensive force fields or the like.
Speaking of force fields, an installation may have them as well. They tend to take two forms: additional Sustained Toughness, applied on top of the installation’s normal rank, and a Create effect capable of producing shapes of solid force such as walls, domes, or cubes, perhaps even enclosing the entire installation. Unlike added Toughness, Create effects are independent objects, offering cover (and perhaps concealment) for anything behind them. An installation beneath a force dome (for example) cannot be damaged until an attack breaks through the dome and destroys it.
Lastly, an installation may offer ranks of Immunity beyond just the normal benefits of its environmental systems. For example, a mystic site may have inherent protection against malign magic, granting anyone there Immunity to Magical Attack (5 ranks). Similarly, an installation may be shielded against outside intrusion, providing Immunity to Teleport effects (and related Movement effects) for 5 ranks, preventing anyone from teleporting into or out of the installation. The Gamemaster approves Immunity effects assigned to an installation, just like any other.
D20 Hero SRD Installations Article- http://www.d20herosrd.com/7-gadgets-gear/installations