It was sealed however the fuck long ago, maybe there was just a limit to how much they coukd remodel it and/or bringing in construction workers deemed way too risky?
It didnt work on them but that black flash could have been considered super secure
I mean, there wasn't even a metal gate. Or hell, a sign inside. It's kind of a trope to have stuff all be sneaky and mysterious but it's also stupid. IRL sure they certainly don't have any external indication that nuclear weapons are stored in some given depot, there is an outer layer of secrecy (though mixed in with conventional weapons such that it's justified to have it be heavily fortified and defended by scary people with guns, analogy here would be if the monks put in "critical sect items and money!" in a vault there thus justifying securing it with a [public facing reason]). But the weapons themselves are clearly labeled. If anyone gets that far no point in beating around the bush on what there is.
So sure, have some disguise and then magic traps or whatever, but then if someone gets right down to that room anyway have a big freaking sign on the wall
WARNING SEAL OF ANCIENT OUTER GOD ELDRITCH HORROR PLEASE CONSULT MONKS PLEASE in Japanese/English/French/Russian/Chinese/everything just on the off chance someone breaks in that far out of curiosity or getting lured in and genuinely doesn't fully understand what they're dealing with.
But "higher intelligence doing very subtle creeping corruption that waits patiently for its chance far beyond normal human thinking timescales" is rare but cool and scary to see done. Little tiny coincidences and twists that add up, how does one counter that?