That'd be my guess also: https://britishlistedbuildings.co.uk/300017169-thorne-island-fort-angle https://britishlistedbuildings.co.uk/300017169-thorne-island...

That'd be my guess also: https://britishlistedbuildings.co.uk/300017169-thorne-island-fort-angle https://britishlistedbuildings.co.uk/300017169-thorne-island... The British Listed Buildings site has photos of the fort and boat approach with stairs, picking crane, and sloped ladder lift for getting loads from water to gate.