Citadel was the very first robotic hero in his universe, and his AI is said to be almost akin to the mind of a human. He used to be named Bastion, but he was nearly totally destroyed in the first Rikti war. His memory core remained, and when D.A.T.A along with Positron realized that he could be saved, and they took all the steps they could to save the unique AI whom they considered to be a person in his own right. Using Positrons advanced knowledge of power suits and robotics and the donated remains of a Hero known as Horatio's power suit they constructed a new body for him, substantially upgrading his systems in the process.
Upon awakening, Bastion found himself in a new body in a much changed world. He adopted a new name: Citadel, in honor of his upgrade and new improved systems as he considered himself a nearly different being. He was classed as a member of the 'surviving eight heroes' and joined the newly created Freedom Phalanx when it was founded, as he realized that his experience could be put to much use, both as a hero and as a trainer for the next generation of heroes.
During the course of his adventures he came across another android that had an even more advanced AI matrix than him named Luminary. Though he does not fully understand human feelings to this day, he felt something that he did not understand for the first time: the forming of concern and a parental bond for this other robot. He took her under his wing and watches out for her. At first she acted as his sidekick, but has since become a hero in her own right. He still watches over her to this day.
Citadel had his name changed from Bastion due to a complaint from the publisher Marvel that they had gotten the name for the character from Bastion.
Citadels Praetorian counterpart is named Siege.
Citadels AI is so advanced that mind magic works on him, though in unexpected ways. When Sorcerers from the Circle of Thorns cast a mental illusion over the Freedom Phalanx that showed their deepest fears, Citadel merely found himself in a blank world that he could shape to his will: he did not understand fear, so though the spell created a mental universe for him, it had nothing to draw off to shape it, but it did count his mind as being real.