Rhyddings Park, Ossy don't know what it was but we used to climb all over it, also meet people there and you could ambush people as they walked through it with water balloons or eggs, not that I ever did that of course !!!
It's inside the grounds of the park. Seem to remember reading that it was part of a structure that was never finished, but can't find anything on the net about it.
4 links here with pics of it from ossygobbins site.