A person would either be forced, lured, tricked or seduced into a small room where the oubliettes opening awaited. From there it was not so long a drop to kill you outright but long enough to ensure you could never climb back out. What greeted you was a room with only the one entrance/exit - the hole you tumbled down.
However, Leap Castles Oubliette was even more nasty, it had wooden spikes lined at the bottom so at the very least you would be quite injured... and with the hatch closed over it was all you could do to wail and cry until you succumbed to deaths embrace.
Leap Castle is haunted by many spirits, many of which no doubt saw their physical life ended in the oubliette.