In the video game "Fallout 3," the Citadel is located near the west end of the Arlington Memorial Bridge in the ruins of Washington, D.C. It is roughly southeast of the game's starting location, Vault 101, along the west side of the Potomac river. Players cannot access it until they have completed certain quests.
Players must complete the quest "The Waters of Life" before being allowed to access the Citadel. This quest is part of the game's main story branch. The door to the Citadel is between the River Entrance and Mall Entrance of the Pentagon ruins, but it is heavily guarded by the Brotherhood of Steel, and they do not open it until an event in the plot causes them to do so. The player cannot fight his way in because there is no means of manually opening the door, and it is the only entrance to the Citadel.
To reach the point at which the door can be opened, players have to complete the quest sequence of finding their father, which begins with "Following In His Footsteps" in Megaton and concludes with "Tranquility Lane" in Vault 112. There are five quests in this sequence, but one of them, "Galaxy News Radio", can be skipped. Completing "Tranquility Lane" is necessary to begin "The Waters of Life."