"Phobos and Deimos Castle" (フォボス と デイモス キャッスル, Fobosu to Deimosu Kyassuru) orbits the planet Mars and in the manga, it is the palace of Princess Mars, given to her by Queen Serenity when she was born.
Phobos and Deimos Castle is first mentioned in the manga and crystal near the end of the dream arc. The Sailor Power Guardians direct the Sailor Princesses to call upon their castles to power up Sailor Moon's Holy Moon Chalice. Princess Mars calls upon Phobos and Deimos Castle and the power allows all of the Senshi to take their Eternal forms.