Citadel is one of the signature heroes in the City of Heroes universe. He bears simularities to Vision or Martian Manhunter.
Citadel was the very first android hero. In the initial Rikti onslaught, he took heavy damage, and was almost completely destroyed. With Positron's help, scientists at DATA created a second version of Bastion using the old model's original memory core and the remnants of a power suit, once worn by the hero Horatio. On the team he often plays the role of the tank, taking shots and throwing punches, although this means he is somewhat unnecessary when Statesman is present.
He has taken on Luminary, a fellow android, as his 'sidekick'.
Citadel waits by the tram on Talos Island, and offers a level 25-30 task force against the Council, who have stolen his blueprints and are using them to improve their own robots. It's somewhat repetitive but fun if you enjoy beating up fascists.
An interesting bit of trivia is that his original name was Bastion, but this was changed to represent an 'upgrade in his capabilities'. In real terms, it was probably due to the fact that the Marvel robot Nimrod went by the name Bastion for a time, so it was changed for copyright reasons.