Air vents...

Still trying out films (and cameras...).
Taken on Agfa APX100, or the modern version of it, using a Pentax Z-1, an unusually bulky SLR by Pentax standards, with an old SMC Pentax 50mm f2 lens (negative digitised using a Nikon DSLR fitted with a 40mm lens...). 
I don't really know what I was taking a photo of, but I would think they're air vents as they're on top of what I believe were ammunition stores in Gravesend's New Tavern Fort, which General Gordon was in charge of when based in the town (along with much else !).