This is the room where the British stored gunpowder. Unsurprisingly, the British military didn’t want any little incidents involving gunpowder, so they kept it nice and safely in this room. Next to this room is a little lobby area where those men dealing with the gunpowder would have to change clothing as their uniform and boots might be “contaminated”. They would change into white clothing and canvas shoes, with absolutely no metal present, before they could play about with the gunpowder. The boxes in the storage room would be made of zinc and there was a glass and brass panel between this room and the shifting lobby.
I have no idea what this means, since I’m not an expert in gunpowder (or indeed an expert in anything to do with the military if I’m being honest), but this is an original British sign painted onto the wall.