Ahhhhh. Yeh that all makes sense now (also as to why the wooden walkway is there!), I think calling it an oubliette instantly leads you to it being a deep pit. Not that it matters too much as once you tell the players what they are seeing it is a lot easier, but I think perhaps the soft edges of the pillar make it harder to tell. Perhaps as your light source seems to be coming from the North West direction then maybe have the pillar cast a shadow on the South East side of the wall, if the shadow reaches to the top of the pit edge it will also show the columns height, though it could just be too subtle to be noticeable. Also an option would be making the floor of the pillar the same stone texture as the floor of the courtyard, but that's a design choice

Anyway, it's all moot as there's no real NEED to change any of it, but just thought I would throw in some feedback.