I do find these mysteries compulsive!
I think it is unlikely to be a sewer vent. Firstly, I was looking at Kyn's fort plan on reply #60. It looks like that the works behind the enfilading fire wall ( the proper name escapes me ), do not seem to extend underground to anywhere near the spot for the vent. Secondly, sewer vents I have seen seem to be around the width of a drainpipe, and are often appear similar to large lampposts at ground level. This thing is huge.
I was looking at Smiffy's latest picture with the side view. Though it is blurred, am seeing a substantive two stage structure. Bottom half possibly brick with the upper stage a fraction wider. Looks to me a lot like a water tower. I am struggling to explain why they would put a water tower outside of the fort, when there are so many higher spots on the fort roof, where that could have been sited.
The site of this structure should be telling. It seems to be right outside the fort main gate. It stands alongside both the approach road and a secondary footpath that runs up around the Casemate. Just the spot for something practical and essential?