Folly
There are many follies in St. Annes Park, though unfortunately they are generally thoroughly defaced with graffiti. The worst offending parts in this case are hidden by that tree trunk, but there's still plenty to see if you zoom in.
This one is particularly interesting, because it is a somewhat ruined ruin, since there was once a real bridge spanning the stream in a single arch, leading to a path through a long colonnade of trees to the north. - It seems this is not the case, see comments below...
Many parts of St. Annes Park are quite surprising when encountered for the first time though, and in spring this really is a delightful vista and very evocative, I imagine, of what the artists of such follies were hoping to portray.