What is this?

On our walk today we were walking around the park when I saw a group of men chatting, so I decided to go around the outside of the park to avoid them.
Walking along the path outside I realised the men were on the grass well away from the path inside the park.
I carried on walking & suddenly saw this thing behind the Tree, as I got closer & looked up at the height of it & it made me dizzy.
So I took a photo of this from the other side, still confused as to what it is, no sign on it to say what it is.
Do you have any clue?