Locks (錠前 Joumae in Japanese) are a type of Demon Tool used for security purposes. Arachne uses them in order to protect her throne room in Baba Yaga's Castle. However, they are all destroyed in the end by the combined forces of DWMA and Medusa's forces.
Like its name implies, a Lock resembles a giant rectangular padlock with a large keyhole in it. A glowing light is seen through the keyhole and many pipes and wires connect the Lock to the building in which it is protecting.
|“||We need to break open a number of gates in order to enter the Spider Queen's Room.||”|
—Medusa, Chapter 48
The Locks provide tight security to a building through the use of magic. The thing most useful about Locks is the fact that one can have as many as they want, and they do not need to be even near the place they are guarding, making it harder for intruders to locate the Locks and try and enter the guarded room.
The Locks each protect a gate which is connected via magic to the actual room it is guarding, meaning that no one can enter the room unless all the Locks are destroyed. There seems to be no limitation in the number of Locks one can use (Arachne uses eight). They can also be placed rather far away from the room they are guarding.
However, the Locks themselves, once found, are quite easy to destroy. It is quite easy to enter the protected room if all the Locks are destroyed, which breaks the protective seal upon the room. As in Noah's example, it is also possible to open the guarded room from inside.
The Locks were probably put in place during the construction of Baba Yaga's Castle.
Part in the Story
Operation: Capture Baba Yaga's Castle
The Locks in Baba Yaga's Castle are built into the eight towers spreading out from the Spider Queen's Room containing Arachne. When Medusa, Maka, Soul, Ox, Harvar, Kilik Rung, Pot of Fire, Pot of Thunder, Death the Kid, Liz and Patty successfully infiltrate the castle, Medusa gives them the mission of destroying the Locks in order for Maka, Soul and herself to enter the Spider Queen's Room. She uses Vector Conduct on Ox and Kilik Rung for them to destroy the Lock Tower No. 2 whilst she entrusts Death the Kid in destroying the Lock in Tower No. 8. The rest of the Locks are to be destroyed by Eruka, Free and the Mizune.
After a bit of a mix-up on Free's part, Kid and Free work together to destroy the Lock in Tower No. 8. Kilik Rung, Ox and Kim and their partners work together to destroy the Lock in Tower No. 2, whilst Eruka and the Mizune destroy the rest of the Locks. In the end, all Locks are destroyed, allowing Maka, Soul and Medusa into the Spider Queen's Room.