@The Omnipotent Sphere: The Twenty-One Circles of the Major Arcana are the twenty-one "Big" Mage Circles, all named for one of the major arcana tarot cards. For example, there's the Tower in London, and the Moon in Japan.
As a general note, this is a setting I've worked on for years and there's too much information to put in the OP. If you have any questions about anything, please, please feel free to ask.