Hōzuki Castle

, also known as the, is a large castle that serves as a criminal containment facility located in Kusagakure, that appears in Naruto 5: Blood Prison. Its was Mui, who uses the ultimate imprisonment technique to seal the abilities of his prisoners.

Layout
The castle is situated near a cliff, where beneath is a raging ocean with tremendous currents that will wash up any attempt escapees and drown them. This, along with the imprisonment technique threatening to reduce anyone who reaches a certain distance from Mui to ashes, makes escape virtually impossible. The prison has no regard of the prisoners' circumstances of being sent there, and outside of the convicts' death, allows them to leave only if the village of the prisoner in question sends an official request for release. Apparently, the prison only admits male prisoners, as Ryūzetsu had to disguise herself as a man in order to infiltrate it.

Deep under the castle is an underground laboratory, which is connected to Mui's office at the top of the castle via a stairway hidden behind a bookcase. Inside is where the Box of Ultimate Bliss resides, embedded as part of a wall, which led others to believe it to be a gateway to the box instead; when the box is to be opened, it will rise to the outside garden of the castle. The lab contains various electronics that fit the stereotypical representation of a mad scientist's laboratory, including a table where a sacrifice to open the Box is strapped down to.

While the prison was built under the appearance of holding dangerous criminals, in truth it was for Kusagakure to hide the Box of Ultimate Bliss and use the prisoners, who are no longer being cared by the villages they originated from, as test subjects by having their chakra sucked out in the lab in an attempt to open the box.

Security
Aside from the imprisonment technique and the lethal whirlpools surrounding the fortress, the castle itself has certain death traps set up in case prisoners attempt to escape. Also, prisoners usually tried to prevent trouble, resulting in an usually quiet and calm environment. Most likely due to these factors, the security is lax, if not outright incompetent, as even though the guards performed strip and body cavity searches, Naruto managed to sneak in a pair of shuriken with wires that saved him from a pitfall trap, and Ryūzetsu managed to hide her gender. Furthermore, in the cases where riots are induced, the guards were incapable of restraining the prisoners, and instead relied on Mui's technique to keep them in check.

Prisoners are let out from morning to sunset for daily exercise and meals, and at the end of the day they are instructed to return to their cells for sleep. Regulation 23 states that attempt escapes or troublemakers result in three days in solitary confinement in the disciplinary cell; clause 6 states second time offenders get five days, third time is a week, and a fourth is permanent.

Plot Overview
Naruto Uzumaki was sent here after being falsely accused of attempting to assassinate the Fourth Raikage, and killing jōnin from both Kirigakure and Iwagakure. This was all part of Mui's plot in order to use Kurama's chakra to open the Box of Ultimate Bliss to release his son Muku. Several prisoners, such as Ryūzetsu and Maroi, were in fact sent in as spies to prevent Mui from opening the box again. Ultimately, they failed to prevent that, and when Muku came out, he was corrupted by the box's powers and served as Satori's host. After a brutal battle, Muku and Mui died together, and the prison castle was burned down. The prisoners, being freed from Mui's imprisonment technique, attempted to escape but were recaptured by the Konoha 11. Mui and Ryūzetsu's bodies were buried outside the castle, while the box was cast into the ocean of the island.

Trivia

 * The castle is surrounded by the hōzuki plant, also known as Japanese lantern. In Japan, its seeds are used as part of the Bon Festival as offerings to guide the souls of the deceased.
 * The sculpture behind the security desk is the Ecstasy of Saint Theresa by Bernini, which illustrates a moment where divinity intrudes on an earthly body.