Welcome to the palace of the heartless queen. It is here that the strongest devils & heartless reside along with the queen of the Heartless herself. The palace was built near the seashore of the city in hopes the the turbine generators powering the place would allow all the needed conveniences for those that reside within.
As it stands over the very spot where Sparda sealed away the portal to the underworld, it is safe to assume that the fortress holds not only the greatest amount of darkness any place could ever hold in the realm of light, but the heartless conjured from the darkness are vast in power. It is for this reason alone that the Palace is now known as the Gateway To Hell.
