The Unquiet Castle is Chapter 29 of Tsubasa: Reservoir Chronicles.
Sakura succeeds in snapping the rusty chain that held her to the floor. She then notices that the only thing locking her chamber door is a small stick which she easily removes with a makeshift noose. Successfully outside of her cell, Sakura begins to follow the children. Suddenly, Sakura screams as the children start to enter an opening in the castle wall. Upon hearing her shriek, the children turn from the hole and begin to approach Sakura.
Outside, Syaoran spots a soaked Mr. Glosum with his horse. Intrigued, Syaoran goes to the Mayor's home and asks to know the exact date when each child disappeared. After receiving the records, Syaoran is out in the cold once more. On his walk, he spots Kyle-sensei inside a home tending to a child. The child, after opening his window and peering out, shouts that he sees a black bird. Syaoran, hearing this, looks into the sky. There is no bird. He then proceeds into the home to meet Kyle-sensei and discuss his findings. Shocked at the notion that Mr. Glosum might be behind the disappearances, he ensures Syaoran that the militia will be keeping a close eye on Mr. Glosum.
That night, as a storm blankets the town in snow, a veiled figure carrying a toy kitten makes its way out into the forest. In pursuit of this figure is a man dressed in a black cloak. As the two figures pause near the castle, Syaoran throws the cloak and toy kitten. He then turns and faces Kyle-sensei, questioning why he came to the castle. Kyle responds by saying he came because he was worried about the child he was following. Just then, Mr. Glosum appears behind Syaoran. We then learn that he fell into the river trying to find a method to cross it. Syaoran then compares Kyle-sensei's patient list to the records of the missing children. Seems Kyle-sensei tended to each of the missing children prior to their disappearance. Syaoran then accuses Kyle of hypnotizing the children and stopping the current somehow so the children can cross into the castle. He cites his experience earlier in the day with the child who swore he saw a black bird outside when none existed. Syaoran, laying it on now, compares the Mayor's version of the town history book and Mr. Glosum's. The Mayor's copy has pages torn from it while Mr. Glosum's is intact. Kyle had mistakenly figured that Glosum would never share his copy with anyone, and Syaoran now reveals that the pages missing from the Mayor's copy describe passages to rooms built under the castle.
Kyle-sensei, realizing he has been figured out, snickers as he admits to the kidnappings. Glosum and the militiaman can hardly believe what they hear. Kyle mentions he kidnapped the children to obtain something hidden within the castle in a place that only children can reach. As the group questions what exactly he wishes to obtain, Kyle darts across the water. Before he leaves, however, Mokona tosses a powder made from the fish scale Syaoran found in the previous world, lighting up his every step. Thanks to another one of Mokona's special techniques, they can now step across the hidden rocks to enter the castle.