This is the protagonist’s home base in CCC.
The building is restored and used by the protagonist, and it is the only safe area on the far side of the Moon.
It is a building that was used during a Holy Grail War that took place long ago; it was abandoned after falling into disrepair. The Old School Building is like a submarine floating in the middle of an ocean of malicious information and data.
Regardless of the fact it was abandoned, it was once the site of a Holy Grail War, so it remains a spiritron source capable of supporting the needs of tens of Servants.
The Sakura in the school infirmary uses the school’s spiritron power as a barrier to protect the school and to maintain the fundamental rules and order of a real world environment inside the school.
…To digress, during the development of CCC the setting was going to be “the EXTRA school building being eaten away at by glitches” but there was no novelty in this idea, so we changed to the nostalgic Old School Building.