Giant Hollow Fortress

The Giant Hollow Fortress is a floating structure commanded by the Unnamed Female Arrancar during an attempt to attack Karakura Town.

Overview
The Giant Hollow Fortress floats high above Karakura Town. It is as large as a mountain with two peaks separated by a large skull with red eyes and an open mouth through which the inside of the fortress is accessed. Below the skull, a hole sits in the middle of the fortress a little above the bottom edge.

The fortress is made up of many weaker Hollow merged together around a central core Hollow. Due to the Hollow comprising it being small and weak, the merging process takes time. The outer walls are permeable to the many Hollow who are able to move through the walls and get to the outside of the fortress more easily.

Within the fortress resides the Unnamed Female Arrancar as well as countless Hollow. The inside of the fortress can be accessed via the open mouth of the skull. Inside the mouth is a large corridor with a roof lined with a spine and rib bones. At it's center lies the core Hollow who is confined to a tank of water and is unable to move. The core holds everything together and keeps the fortress afloat. The core is also the main weak point due to this but it is guarded by a large Hollow. Upon the destruction of the core, the Fortress falls apart and is much easier to deal with. Kisuke Urahara was able to destroy the entirety of the fortress once the core was destroyed with his Sing, Benihime attack.