Pleiades Watchtower

The Pleiades Watchtower (プレアデス監視塔, Pureadesu Kanshitō), otherwise known as the Great Library Pleiades (大図書館プレイアデス, Dai Toshokan Pureadesu), is a watchtower created by the Sage, Flugel. It was overseen by Shaula until the end of Arc 6. The tower is located in the Dragon Kingdom of Lugunica's Augria Sand Dunes, and it allowed Shaula to keep a lookout for trespassers who were aiming to visit the Evil Sealing Stone Shrine where Satella is currently sealed. The tower as a whole also serves as a great library, and once entered by someone, the visitor is locked into a series of examinations to gain access to the various floors of the tower.

Following the arc's climax, Emilia completed all three trials, which granted her the authority of the tower's administrator, and she ordered Volcanica to prevent anyone else from approaching the area.

Appearance
From a distance, behind its defence mechanism known as the Sand Time, the watchtower appears as a silhouette that never changes in position, shape or form, no matter how close you try to approach it. After crossing the Sand Time, the tower appears from its exterior as an extremely tall, circular shaped building made of stone. The tower appears to be over a couple of hundred meters in height, and it has a balcony that juts out from one of the floors. Furthermore, a huge door can be seen at the base which leads to the inside of the tower.

Its first line of defence is the aforementioned mechanism known as the Sand Time. It is an invisible force field that twists and bends space, making it impossible to pass through it. It acts as a deterrent for everyone who wants to approach it, and, upon being sent, not even Reinhard van Astrea was able to find a way to pass through it. Despite that, there is a single flaw in its defences; a rift which can only be exploited when a sandstorm occurs. Only then can one pass through Sand Time and reach the actual tower, provided they can avoid Shaula's Hell's Snipe and countless deadly Demon Beasts residing near the tower's outer walls.

At the end of Arc 6 after relentless attacks from demon beasts, the three Sin Archbishops of Gluttony, and a massive attack from Volcanica's breath, the tower was nearly destroyed and has massive gaping holes around the upper floors.

Following the climax of Arc 6 and the liberation of the Watchtower, the Sand Time barrier disappeared, Shaula was neutralised, and Reid's soul was been shattered, leaving the Divine Dragon as the only actual living creature in the Watchtower. Before her departure, Emilia instructed the Dragon to guard over the tower and attack anyone who approaches, only on the condition that the Dragon may not harm anyone.

History
The tower was designed by Flugel 400 years ago as a means to prevent intruders that seek to reach the Evil Sealing Stone Shrine where Satella is currently sealed. 400 years ago, he commanded Shaula to stay in the tower and kill anyone who approached unless they were himself. He also put her in charge of upholding the tower's rules. Flugel also likely designed the examinations and mechanisms inside the tower which protect the various floors within them.

Throughout the four centuries of the Watchtower's existence, numerous attempts to besiege the Tower and resurrect the Witch were made by the Witch Cult, though none of them were successful as they were likely obliterated by Shaula and numerous Demon Beasts.

Both Emilia and Natsuki Subaru noticed that Echidna was related to the tower to some extent. For example, Subaru learned that Eridna mostly knew nothing about her past except for how to get to the tower, and Emilia noted similarities between the Trials at the Sanctuary and those at the tower.

Floors

 * Asterope (アステローペ, Asuterōpe): Asterope, known as the sixth floor, is the basement floor of the Pleiades Watchtower. The room is circular in shape, and quite spacious, approximated to be about 200 or 300 metres in radius. The room's walls are adorned by magical lamps and luminous moss which provide it some illumination. A spiral staircase located on the outer edge of the room leads up to Celaeno.
 * Celaeno: (ケラエノ, Keraeno) Celaeno, known as the fifth floor, is the ground floor of the Pleiades Watchtower. The room is described as a giant hall with no distinguishing features, except for a large door of around about 10 metres in height which leads to the outside of the Watchtower. The door is operated by some sort of mechanical opening system that reacts to touch. Celaeno is connected by the large spiral staircase leading up to Alycone, and one down to Asterope.
 * Alcyone (アルキオネ, Arukione): Alcyone, known as the fourth floor, is the first floor from the ground of the Pleiades Watchtower. The floor has a multitude of rooms, which include Shaula's living quarters, a dining hall which has a well in it to draw water, along with the Green Room (緑部屋, Midori Heya), which is inhabited by a Spirit that bestows healing upon its occupants. The balcony that juts out of the Pleiades Watchtower can be accessed from Alcyone. Further to the great spiral staircase that connects it down to Celaeno, two further staircases are in this room. One which leads to Taygeta, and one that leads to Electra.


 * Taygeta (タイゲタ, Taigeta): Taygeta, known as the third floor, is the second floor from the ground of the Pleiades Watchtower. In its initial state, the room is completely white and only has a single monolith right after the stairs that lead up to the floor. Once touched, the room enters its examination state, and is covered by a series of monoliths which represent stars and their constellations. The first exam takes place on this floor, whereby its challengers must touch the brightest of the hero destroyed by Shaula. The correct answer to this challenge is touching the monolith that represents the star Rigel. Once completed, the room transforms into the Taygeta library, which contains the books of the dead. Taygeta can be reached by taking a set of stairs from Alcyone.
 * Electra (エレクトラ, Erekutora): Electra, known as the second floor, is the third floor from the ground of the Pleiades Watchtower. The floor in its initial state only has a sword lying in the middle of its room. Once touched, the second examination of the Pleiades Watchtower commences and Reid Astrea materializes. The challenge on the floor is to make Reid yield a step in combat. Electra can be reached by taking a set of stairs from Alcyone.
 * Maia (マイア, Maia): Maia, known as the first floor, is the fourth floor from the ground up, of the Pleiades Watchtower and is right at the top of the tower, situated outside. The floor takes shape of a large circular space, 100 meters in radius, with six pillars placed around the outskirts of the floor and a giant pillar located at the very middle of it. Overseen by Volcanica, who is idly leaning against the giant pillar in the middle, the final examination takes place on this floor. Maia can be reached by taking a set of stairs from Electra, providing the person who does have beaten its challenge.
 * Merope (メローペ, Merōpe): Merope is known as floor zero of the Pleiades Watchtower. The existence of this floor was hidden by Shaula, and with the exception of admitting that it was built after Flugel's departure, she refused to give information about it.
 * Underneath the Tower: An underground area underneath the sand dunes that connects to the Pleiades Watchtower. The area is not illuminated, and it is full of rocky tunnels and caves. There are two layers to this area, and Gabaou infest the area, especially on the left path. In addition to that, a strong miasma emanates throughout the right path; several obstacles which Natsuki Subaru described as "Doors" block the path the deeper you traverse through this path. With Subaru has been able to open three of them, it is thought that there are seven in total, with the miasma growing thicker the more doors that are opened. The miasma originates from a door that stands without support in the middle of the above layer. The door transports a person back to the Pleiades Watchtower if they reach a door that they cannot open.
 * Sand Shrine (砂宮, Suna Miya): A term used by Shaula to refer to what Subaru had gotten near to when she found him down in the underground area. She mentioned that it was reckless of him to be there without having all the keys ready.

Rules
There are five rules which challengers must obey whilst in the tower. In the case that a rule is violated, then the Stars Keeper Shaula may turn into her scorpion form which she describes as being a "bloodthirsty killing machine" and eliminate them all. The rules are listed as follows:


 * It is forbidden to leave the tower without completing the trials.
 * It is forbidden to violate the rules of the trials.
 * It is forbidden to disrespect the library.
 * It is forbidden to damage the tower.
 * It is not forbidden to destroy the trials.

Trivia

 * In Japanese, the name "Subaru" refers to the same cluster as the Pleiades.
 * All of the tower's floors are named after the seven sisters of the Pleiades from the ancient Greek religion.
 * In the Web Novel, the underground area had only one door and seven jewels instead of an unknown amount of doors. When Subaru approached the door, four of the jewels lit up.