Location from Attack on Titan by Hajime Isayama
A ruined castle where Survey Corps soldiers were trapped by Titans that attacked at night — an impossibility that meant someone was controlling them. Where Ymir revealed her secret.
Utgard is a ruin that became a siege. An old castle in the territory between Wall Rose and Wall Maria, it should have been empty. Instead, a squad of Survey Corps soldiers took shelter here during a nighttime operation and found themselves surrounded by Titans — Titans that could move in darkness, which shouldn't have been possible. The castle's crumbling walls became the only barrier between the soldiers and death. Supplies were nonexistent. Stairs collapsed. People died holding doorways. It was here that Ymir — the small, sharp-tongued cadet that nobody paid much attention to — transformed into the Jaw Titan to save Historia, revealing that she had been hiding a Titan power the entire time. The castle didn't survive the night. Some of the people inside it did. Barely.
Also known as: Utgard, the ruined castle, Castle Utgard