Guild crafting room for explosives that becomes unreachable once the saferoom entrance is collapsed.
Carl lists the bomber's studio alongside his Sapper's Table as facilities Justice Light will lose when The Bait Shop is collapsed and the guild saferoom goes offline. The eagle tells him it doesn't matter—there is nothing left he cannot do on his own—and Carl accepts the loss, keeping the ruined shack inside The Castle under heavy guard in case it ever needs to be repaired in an emergency.
One of the guild's dedicated crafting rooms, used for building explosives.
Explosives workshop sacrificed with the guild saferoom access.
Also appears in books you've already read — spoiler-safe to their endings: