At Gillette Castle State Park in East Haddam, CT.
This lower room was used by the builders during the construction of the castle, and later as a workshop where Gillette designed and made the theatrical elements of his home. It's just off the entry hall and was not apparent to his guests. It's behind glass so the public won't disturb the original items in the room.
On a personal note, a branch of my family helped build the castle in the 1910s, according to my father. I haven't been able to find any reference to them, though. Their name was Bannon, if anyone comes across a reference they can point me to.