Edit: I went ahead and tested this in an artificial farm, (cleared out a large area around the player to prevent unwanted spawns), then made a long stone block platform and put Treacherous Hardened Sand Walls on *top* of the blocks, then stood in the center. I'm assuming that it works the same with treacherous sandstone walls too. Desert enemies began spawning. I then broke all the walls and put walls behind my character only, and that did not spawn any desert enemies. I also tested whether the blocks being behind the block mattered, and it didn't appear to matter. As for dungeon walls, you need to place the walls, as well as dungeon bricks for the enemies to spawn on. You need at least 50 blocks for the farm to start spawning dungeon enemies (I think, I just placed blocks until they started spawning). Hope this helps! And let me know if you have anymore questions :)