"Io Castle" (イオ キャッスル, Io Kyassuru) is the home of Princess Jupiter, and it was given to her by Queen Serenity when she was born. It orbits around the planet Jupiter and appears in the manga and Crystal.
Io 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 Jupiter calls upon Io Castle and the power allows all of the Senshi to take their Eternal forms.