Selkie Aquarium

Selkie Aquarium (セルキー＝アクアリウム) is a mobile fortress, owned by England.

Principles
Selkie Aquarium is apparently a submarine-type mother ship for Magicians carrying out underwater activities.

English Civil War Arc
Selkie Aquarium was one of several Selkies still operating after the Knights' attack during coup d'état. It and several other mobile fortresses participated in a bombardment directed at Buckingham Palace, attacking from the bottom of the ocean near Dover.