At the end of the hallway, the players pass through a simple opening into a circular room. The room has a stone floor, and walls of bronze which gently curve inward at the ceiling, rising to a point in the center. The walls may be engraved or otherwise adorned, but the room itself is empty. If the players decide that this is merely a dead end, it is important that they be able to go back through the dungeon and take an alternative route.
If the players choose to tap the walls, they will find most of them to be solid. However, there are definite hollow areas which echo when tapped. Closely examining the floor where the ceiling meets the wall will reveal that the bronze walls are fitted into a groove in the floor which extends all the way around the room. This groove is most obvious where the players entered the room. However, given the generally poor condition of the floor, characters will not notice this groove unless they pay special attention to that doorway.
In fact, the bronze cone has special ball bearings beneath it, and can be turned freely. It is heavy, But a concerted effort from a strong character, or two characters with strength 10+, is sufficient to slowly turn the room at a speed of 1/4 rotation per game turn (10 minutes). More characters working together will be able to turn the walls proportionally faster. As the wall turns, the opening moves, revealing previously hidden passages behind the bronze wall.
The danger, of course, is that by turning the cone the players are cutting off their means of escape back through the dungeon. Unless they have enough people to turn the bronze cone very quickly, they will be forced to face any dangers they reveal.