I know it’s been awhile since the My Little Pony movie came out but there’s something I still don’t understand: Why exactly did Celestia have a room in her castle specifically designed to steal her power?

I mean surely the giant flaw of having it would have come up during the design and building phase. And someone had to sign off on it.

To be honest I imagine the conversation going something like this:

Celestia: Ah, it’s finally complete. A new castle for a new era wouldn’t you agree current captain of the royal guard?

Captain of the royal guard: Yes it is a grand castle should be easy enough to defend.

Celestia: Good. Not a single flaw in its defences?

CORG: N-no.

Celestia: You, uh, you hesitated there. Is there something I should know?

CORG: No, it's virtually perfection, like 99.99%.

Celestia l: Uh, okay, wouldn't be doing my job if I didn't ask what's the 0.01?

CORG: Well, I mean, there's this little hole. It was kind of an aesthetic choice by the architect. And if you put the Staff of Sacanus into this hole, it steals all your power.

Celestia: Whoa, whoa, whoa, whoa, whoa! That sounds like a pretty big design flaw, then.

CORG: No, no, it only really works if four alicorns stand in proximity to it.

Other guard with them: Well, with Nightmare Moon gone Celestia’s the only one left.

CORG: Exactly. And even to get to it, you’d have to get past all the guards plus you’d need the Staff of Sacanus which has been lost for centuries. It's not a big deal.

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It probably has nothing to do with the room, castle, or architecture. It was a room that happened to be large enough to inscribe the magical sigils into and they had time to prepare it while Fizzlepope went about retrieving the fourth alicorn.