Q: What is the role of the eyeball that appears in Blood Fort Andromeda? Is it the Backbeard's? <Aisaka Riku>

Nasu: Everyone forgets this, but that is Medusa's eyeball. It seals other people within oneself, hence why it's called Blood Fort Andromeda [Outer-Seal, Blood Temple]. The entire school building has been pulled into the eyeball of a monster.
Takeuchi: Like a Conceptual Barrier? If it goes too far, won't it turn into the Shapeless Island?
Nasu: Oui, Monsieur. The devil's den that Perseus escaped is invoked by Shinji, it is meant to be ironic.

Fate/Stay Night Animation Material I; Q&A, pg. 42