When we first enter the Black Armoury and meet Ada she tells us that this place is not meant for Guardians. Now hold on just a damn second, if the Black Armoury is not meant to be visited by guardians, then why the frick is it located in the -blam!-ing tower, literally the place where guardians gather. Secondly, how the -blam!- does a golden age armory exist in the -blam!-ing tower, and we guardians, you know, the ones who own the the tower, not know about this? Like it's literally located a 2 min walk away from the Vanguard Commanders.... How the hell does Spider know about this place in the tower and we don't? How?......
Arguably, it's because when the Armory was built, the Guardians were in the old Tower, which was recently trashed by the Cabal. Then, the Guardians moved to the Black Armory's doorstep, ironically unaware that they were taking up residence next to an ancient Golden age era death factory.