@richardjjs I believe the room with "the little lamp unit" is the powder room. They didn't want to have a lantern with an open flame in the same room where they were working with large amounts of gunpowder.

I believe those racks you noted would hold individual charges of gunpowder ready to be used in the cannons. I believe you could come down a ladder to get into the area where H is displayed, then go through the door into the space marked with I. Then once the door to H is closed, you were allowed to open the door from I into the powder room. Also, I believe they sheeted the powder room with copper so there was no iron or steel to cause a spark.