The dungeon is 13"x13"; the outer ring is 2" high (4 rows of fieldstone) but sits ¼" lower than the inner ring because it is flush with the foamcore while the inner ring, also 2" high, sits on the foamcore and the ¼" cobblestone. This gives the inner chamber decent wall height for furnishing but allows the players to easily see the figures in the hall around it.

I decided to make the inner chamber a secret one, accessed only by a hidden door. It's so well hidden that I have to feel around for it myself!!

I'm going to make the room into a Drow forge and armory; the metal is only hammered in the wee hours of the morning, sending a ghostly and mysterious rhythm throughout the dungeon. Hence, the name The Ghost Anvil. The forge will be located over the branch of lava leading to the hole (to be filled I don't know how yet); that the lava provides the heat for the forge allows me not to deal with ventilation problems associated with burning wood or coal ;)