The vampires and everyone else called the world inside Arad's stomach a 'Small' world, because it was objectively smaller than the outside world. But D found that description quite misleading; this place was by no means small. And get this, it's growing.
While the Castle of Alina might serve as the living and breathing capital of the empire, very few knew that the actual heart, the core of the whole empire, hid safely inside the Emperor's stomach.
Here is the place where most of the powerful spirit queens lived, where the countless vampires trained, and organized their operation. This is where the countless war mages of Ruris's empire were testing their spells. D had even seen several angels flying around, taking care of whatever business the Mira and Eris churches needed.
Most of Arad's hoard was hidden here; the kingdom's whole wealth was also stored here in vaults and guarded by the harrowing vampire dragons that Arad had. Even the merchants held most of their deals here.
