Altera’s Cage

A prison for the Main Character, hastily constructed by Titan Altera.
Although from the outside, it looks like there’s ample space to squeeze through the bars, it goes without saying that even the “gaps” are barred shut, so breaking free is impossible.
The reason why Titan Altera created the cage is revealed in the game.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA