Well, the area under the trap door seemed to be accessible, it's the basement to the slave house, and there is another entrance, somewhere east of the desert town city, if you leave it through the north and go around it, on the right side, and there is a guy in there that can be knocked out for a map to the slavers ' s hideout. The hideout has a dev lock now, so not very interesting, but that other entrance in the basement, that wasn't marked on the map, nor was it any clue for it, I found it randomly, by reading some comments and looking at some screen shots, and I had to "poke" the wall, just like with the entrance of the Succubus tower, the first time, though I have no idea what clues should I have looked for to guess how to get there, or perhaps my sen stat is low, I don't know.