There are defense turrets on the tower... And it's rare but the Warlock Vanguard Ikora Rey will say "The Fallen are trying to invade, prepare the defenses!" I'm not saying there isn't some underlying threat in the Tower but the Guardians aren't truly evil. Plus the Darkness invaded our Solar System after the Golden Age of peace the Traveler provided for us so once again we weren't the cause of this war between the Fallen, Cabal, Vex and others controlled by the Darkness.