"The only way in or out was from the air..."The Citadel is a location in The Last Guardian. It serves as the final level of the game and is the lair of the evil Master of the Valley.
The Citadel is a huge tower that stands out from all the other buildings in the Nest. While other towers are crumbling, overgrown, and made of darker stone, the Citadel is strong, white, and towers above all else. The Citadel is the only building tall enough where, on the roof, one can see over the Nest's impassable natural walls. The Citadel, rather than having the defined lines of bricks, appears seamless and smooth. Its general shape is reminiscent of a King chess piece.
With Trico unable to fly, the Boy realizes that the best way to escape the valley and its insurmountable walls is to get to the top of a massive white tower in the Nest. The bridge partially collapses under Trico, and the way is blocked by talismans. The boy navigates rotting wooden beams and narrow stone ledges to get to where he can remove the obstacles. However, a huge army of Armored Knights ambushes the Boy. Suddenly, Trico's love for the Boy overcomes his instinctual fear of the barriers, and he charges to the rescue, further damaging the bridge. The two barely escape death by miraculously gliding to a nearby tower with Trico's partially healed wings.
The bridge no longer an option, the two get to the top of a nearby tower and glide right to the front gate of the Citadel, where Trico lays waste to the Knights guarding it. Navigating the interior, they find the Master's chamber and gain access to the wide, open roof of the tower. Before they can escape, the Master summons a large group of Armored Tricos, who attack and subdue Trico. The Boy manages to destroy the Master, causing the enslaved beasts to fall from the sky and the Citadel to partially crumble. Although wounded, Trico amazingly takes flight, just clearing the mountains, and returns the Boy to his village.
Some time later, the Boy, now a man, activates the mirror. In the final cutscene, it is shown that the Citadel still stands proudly above the valley, although the evil hidden within is gone forever.
The Citadel has two exterior locations. First is the front gate and courtyard. As well as having a big portcullis, a large number of smaller doors allow the rapid deployment of Knights to defend the courtyard. Next is the rooftop, which is wide and flat with a broadcast "antenna" which the Master uses to control its subjects, as well as a shrine where the Armored Tricos deposit the Master's sustenance: children.
Once past the portcullis, there are curving hallways, large dome chambers, a room with a bottomless hole, and two elevators. Higher up there is the Master's chamber, suspended in a cavernous room carpeted with broken barrels. Above the Master is a spinning fan and then the roof access. Most of the rooms are cold and misty, glowing with a soft blue light.
The only way to access the building on foot is by a rickety wooden bridge that can barely support Trico's weight (in fact, it almost completely collapses when he walks on it). The bridge is barred by two talismans, and the bridge allows for the use of long-range attacks on anyone who approaches. The portcullis can only be opened with the mirror.
The Citadel hosts a large army of Knights, and likely serves as their headquarters. They can rapidly deploy to any part of the tower through the mysterious blue doors. Armored Tricos can also be called to defend the tower.