Any ideas?

On Monday I have to lead a walk for the Walking for Health and if I pass muster I will be a qualified leader. So I thought I better recce the walk this morning.
It starts by walking all around our leisure centre which used to be my playground as a child; 20+ years before the leisure centre was built. There was a slide, seesaw and roundabout but also bunkers left over from the German occupation to clamber over.
I don’t remember this building but I think  it was probably behind a wall or fence or something. No idea what it is. My only possible guess is that it has something to do with ropes because it is fairly close to a ropewalk.