Utgard Castle

Castle Utgard is an abandoned castle located within Wall Rose, close to it's perimeter.

Story
The castle is first discovered when two squads from the Survey Corps come across each other while searching for a breach in Wall Rose. The decision is made to take refugee within the nearby castle, and rest until morning before resuming their patrol. The group includes senior members Nanaba, Gerger, Henning, and Rene as well as several rookie members under their command -- Bertolt Hoover, Reiner Braun, Connie Springer, Christa Renz, and Ymir.

Inside, they discover evidence of recent occupation and guess bandits must have used it, noting several mystery crates full of supplies. The group gathers in an intact room, and attempt to get some rest while things are quiet. However, Connie brings up the mystery of his village and an argument breaks out when Ymir mocks him. The senior officers set up a watch on top of the tower, and prepare for a long night.

At some point, Ymir sneaks away to look for food. She is confronted by Reiner, and they have a brief discussion before he notices her ability to read the strange characters on the supplies they found. But before he can say anything more, the castle comes under attack by Titans able to function normally during night. As the group gathers on the tower, they spot the Ape Titan walking past the castle and climbing Wall Rose. The four senior Scouts go into battle, using the tower to successfully fight against the first wave of Titans. Several smaller Titans break into the tower, and are fought off by the rookies at the cost of Reiner being seriously injured.

Rene and Henning are both killed when the Ape Titan hurls several chunks of the Wall at Utgard's tower. It then summons more Titans to attack the castle, before climbing over the Wall into the lost territory of Wall Maria. Nanaba and Gerger continue to fight the Titans until their fuel and blades run out, at which point both are devoured by the horde. Without any weapons or method of escape, the rookies can only wait atop the tower until dawn and prepare to die.

However, at dawn Ymir makes the decision to reveal herself to be a Titan Shifter and jumps off the tower to fight the other Titans. During the battle, the tower is badly damaged and she prepares to sacrifice herself to prevent its destruction. Christa encourages her to destroy the tower instead, and Ymir tears it down before rescuing the others. The remaining Titans are buried in the rubble of the castle, but manage to climb back out and overwhelm Ymir. She is almost completely devoured, but Hanji Zoe's squad arrives to rescue the survivors from the ruins of Castle Utgard.

Trivia

 * In Norse mythology, Utgard is the stronghold of the Giants.