Gibb

Gibb is, technically speaking, underground.  The magical coverings over it appear to be solid hillsides from above ground, but within Gibb, they are clear and allow in light and for the residents to see the sky and scenery.

Gibb is a predominantly human city, with portals to Earth.  Some lockettes do live there, but they face strict approvals and evaluations from the Dimensan government before being allowed to do so, and depending on the strength of their summons, may be required to wear a device on their collar that inhibits their ability to summon them.  There are guards beyond Gibb who do not face this requirement - but they are kept unaware of what they are guarding, and are typically imprisoned or killed if they discover it, in order to assure the safety of the humans of Gibb.