The 14th upper floor of Damavand-Volcano. Unlike the other floors, including the 1st-to-13th and the 15th upper floors and the underground 1B-to-7B floors, there is technically nothing here for the invaders to make use of.
How this is possible is thanks to Flint's simply nasty layout of the Dungeon. The invaders have to start at floor B1, go down to B6...then have to climb up to floor 13. The reason for how Flint managed to make the invaders go through floors B1-to-B6 twice is that he divided the large, 5-kilometer-big floor into several passages that are separated from each other by walls or large pools of magma.
As for the 14th floor, Flint made it so that people can simply pass it to reach the 15th floor, the top of the volcano...where they have to descend a path to reach floor B7, the real final floor of the Dungeon.
