I don't really know how the Church can deny that there is a basement in the Fort Harrison. It's amazing!
I was there as a student in 1976, and several other times in later years. In fact, I was there during the time when it became known that the United Churches of Florida (which originally bought the Fort Harrison) was really a front for the Church of Scientology, and the shit hit the fan (to coin a phrase) in Clearwater, Fla.
At that time, there were dark, grungy rooms under the Fort Harrison which I had frequent access to. They were used for folder archives, to store pc folders that were not currently needed.
There were also doors into other rooms under the Ft. Harrison which were constantly guarded, and access to which was closely monitored. Being the curious sort, I asked a couple of staffers what was down there, and they told me that people on the RPF were lodged down there while they did their amends.
At least some of these basement rooms were later opened up and remodeled. When I was last at the Ft. Harrison around 1983, those rooms were in use as the internship courseroom, but the space used for the courseroom was by no means as large as the entirety of the Ft. Harrison. So there was a lot of space down there which was only partially taken up by the courseroom.
It's nuts to deny that such an area is there! Anyone who goes to the Ft. Harrison can go down into the Intern Courseroom area (if that's still what they use it for), and see that there obviously IS a basement in the Ft. Harrison.
There were also other places in the Ft. Harrison where people could be kept incommunicado. I have personal knowledge of a small storage room (about 6' X 8' X6') off a stairway leading out of the galley in which a sea-org member who had had a "psychotic break" was kept for several weeks while they hoped she would become sane enough that they could release her to her family without causing too much of a flap. I was a close friend of the lady who was assigned to bring the captive her food and spend a very few minutes each day with her. She was not allowed ANY company at ALL, except for these brief visits. I wasn't around long enough to know the ultimate outcome of that particular instance of captivity, but from other stories I have read, her period in the "dungeon" was not so very unusual.
So I have little reason to doubt Dennis Erlich's contention that he was similarly imprisoned for a time.