Cystern Access door won't budge

[SophiaD]

Hi all, I'm trying to get through the cystern access door in the northern cell area. I've removed the padlock with the acid and hammer, yet it still won't open. I looked around and didn't find anything to suggest there was anything else I needed to do to open it, and upon inspecting the door more closely noticed that the very edge of it seemed to be inside the wall. Am I right in thinking this is a glitch or is there something else needed to open the door? Thanks