The house elves are categorically forbidden to enter this area, on pain of never being able to feel pain again. The same applies to ever mentioning or discussing it. The family, with the exception of Arcturus, are unaware of its existence. Arcturus himself stumbled across it when he levitated a particularly unwieldy cask of 1905 Amontillado, and accidentally struck the wall, which, being only a sort of papier-mâché, crumbled. There is therefore only the one point of access, behind the Port Barrels, and now the wall is solid and requires a sign, a password and a voice match to permit entry.
Once more than about 20 meters beyond the door, the passageway opens up into a high vaulted series of seven rooms, each roughly 20 meters by 12. They currently contain the weapons and devices which the Ministry started to develop and then deemed too dangerous to allow them to go into production, along with various "mad" ideas which generations of Rookous have sponsored and squirrelled away. Archie gets a chill thinking about this place, the relict of innumerable beatings, and is unlike to venture hither.