The "Fortress" has been keeping us safe, though they aren't doing this for no reason, "They" expect payment. Either natural resources, like metal and wood or things like food, water, clothes and other necessities for the people in each "circle" of the Fortress.
But not everyone can pay these "taxes" just like that, only the first and second circle can. The people within those circles are either descendants of "SHIELD" aka the founders of the Fortress or high rank military. The people who are neither of these, are the ones in the third and fourth circle, aka "SWORD". I am one of these people. In SWORD you are required to be "the sword for the shield". For protection they ask you to explore beyond the walls of the Fortress and bring the company under the SHIELD you work for, goods and anything that could be valuable.
That's what the Fortress' all about.