Ventris's secret fortress located somewhere in the far reaches of the Milky Way Galaxy. It is where Ventris secretly dumps his kidnapped teenage girls and eats them for breakfast, lunch, and dinner.
Nobody knows the castle's exact location; the castle actually teleports to another random location within the Milky Way Galaxy every 10 minutes, making pinpointing its exact location nearly impossible. The warp devices used to teleport the Space Castle originated from now lost Space Dragon technology, with significant modifications and improvements. The dark lady herself installed these upgrades, made from unknown, forbidden technology she developed herself.