The Pleiades Watchtower (プレアデス監視塔 Pureadesu Kanshitou) is a watchtower created by Flugel that is overseen by Shaula. The tower is located in the Kingdom of Lugnica within the Auguria Sand Dunes, and it allows Shaula to keep a look out for trespassers who may be aiming for the Evil Sealing Stone Shrine where Satella is currently sealed. The tower as a whole also serves as a great library, and once you enter you are locked into a series of examinations to gain access to the various libraries on its floors.
From a distance, behind its defense mechanism known as "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 metres 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.
The tower was designed by Flugel 400 years ago as a means to have a lookout against 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 inside the tower, which protect the various libraries within them.
Both Emilia and Natsuki Subaru noticed that Echidna was related to the tower to some extent. For example, Subaru learned that Echidna (Artificial Spirit) remembered not much but how to get to the tower, and Emilia noticed similarities between the Trials at the Sanctuary Barrier and those at the tower.
- 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 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 Alycone. Further to the great spiral staircase that connects it down to Celaeno, two further staircases are on 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 examination takes place on this floor, whereby its challengers must touch the brightest of the hero destroyed by Shaula. The correct answers to this challenge is touching the monolith that represents the star Rigel. Once completed, the room transforms to the Taygeta library, which contains the books of the dead. Taygeta can be reached by taking a set of stairs from Alycone.
- 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 Alycone.
- Maia: (マイア Maia) Maia, known as the first floor, is the fourth floor from the ground of the Pleiades Watchtower and is right at the top of the tower, outside. Presided by Volcanica, 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 has 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 she refused to give information about it.
- Underneath the Tower: An underground area underneath the sand dunes lies beneath 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. The miasma originates from a door which stands without support in the middle of the above layer. Seven jewels adorn the door, of which four shine for Natsuki Subaru. If you enter the door without holding something that acts as the key, the door transports you back into the Pleiades Watchtower.
- Sand Shrine (砂宮 Suna Miya) A term used by Shaula to refer 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.
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 libraries.
- It is forbidden to damage the tower.
- It is not forbidden to destroy the trials.
- In Japanese, the name "Subaru" refers to the same constellation as the Pleiades.
- All of the tower's floors are named after the seven sisters of the Pleiades, from mythology.