As its name suggests, the Hidden Fortress is concealed quite well on the Legacy. Inside its walls are cells used to contain prisoners of war and torture devices for interrogation. In the back of the fortress, there are documents containing records of Vaclav Bolud's previous endeavors.
After Shirley Fennes is captured by Vaclav, she is imprisoned in one of the cells of the fortress and discovers that her shell is suddenly glowing. She is relieved that her brother, Senel Coolidge, is alright. She then hears a voice from the cell beside hers. The voice introduces herself as Fenimore Xelhes, who tells Shirley that other Ferines, who were captured in her village, were killed. She further explains that she has a twin sister, as well as that she has been praying for the Merines to arrive. Shirley then notices Fenimore in an extremely injured state due to the torture from the guards. While Shirley pleas with the guards, the Terrors arrive and explain that they have been given orders to take direct custody of Shirley. Stingle intervenes and manages to spare Fenimore from the Terrors. Upon seeing Shirley, Fenimore is disgusted by the fact that Shirley was living with Orerines. Suddenly, the Terrors are ordered to take Shirley to the Ruins of Frozen Light.
Later in the story, Walter Delques leads the group to Vaclav's fortress after learning of remnants hiding there. As they continue, the group finds instruments of torture and prison cells that were in use until recently. At the back of the fortress, they find a library filled with books containing information regarding human test subjects. According to the book Jay reads, Vaclav has been carrying out an experiment to create an artificial Merines and using countless Ferines to do so over the years. Senel is angered by this, and Will Raynard suggests taking the notes with them so no further problems will arise.
Walter explains the true reason he brought the group to this fortress: to show them Vaclav's crimes to them and eliminate Senel for leading Shirley astray. He informs them that Shirley is now re-attempting the Rite of Accession ritual to bond with Nerifes. As a fight between Walter and the group is about to commence, Walter's Teriques appears and informs him that an army is making its way toward the ritual site, which prompts Walter to leave. The group attempts to pursue him, but they are blocked by a pair of "Galfs" wearing gold and silver armor. Upon defeating the beasts, Jay finds a book titled Operation: Merines Capture. Senel demands the book, and Jay willingly gives it to him, in exchange for being able to see the contents later. The group then makes their way to the Altar of the Sea.
During Chloe Valens's quest, the group travels to the Hidden Fortress in order to find Elsa Alcott, who leaves town to search for her father. Upon entering, they find Elsa limping forward. Chloe scolds her for leaving on her own, even after she told her not to. Senel is somewhat concerned about releasing Elsa on her own, so the group decides to allow her to accompany them in their search for Arnold. As they reach the torture room, Chloe decides to let Elsa rest, while Moses Sandor, Jay, and Grune scout ahead. While resting, Elsa begins talking in her sleep about Chloe, waking only to be embarrassed by this. Elsa explains that Chloe has become a bit of a role model for her.
Moses, Jay, and Grune then return from their scouting and speculate that Arnold may have ventured further in the fortress. As the group reaches the library, they find Arnold wounded. As Senel and Will assist Arnold, they find what appears to be a snake tattoo beneath his sleeve. After healing him, Chloe draws her sword, and the others spot a "Chariot Gaet". After dealing with the gargantuan beast, it suddenly returns to its feet, but Arnold finished it in a single blow. Senel is amazed by the technique, and Arnold explains that what he is after is inside a nearby locker. Norma Beatty attempts to open the locker and fails, but Arnold manages to open it himself. After Arnold apologizes for troubling everyone, the group leaves, as to avoid other monsters that may be inside. Chloe then realizes Arnold's true identity.