My guess is the walls are around a hole in the ground where a building is being built and the contractor provided a "balcony" for the curious sidewalk superintendents passing by to view the project. It even has a an awning for rain protection and it looks like seats too. Thus the name "Ye Overseers Observatory".

Addendum...
I didn't notice that there is a reference to "Sidewalk Superintendents" on the sign until after I posted the above comment. That term was passed down to me from my father many years ago and it has stuck with me. There is also a new term for me on the sign too..."Excavation Enthusiasts". I will have to add that one to my vocabulary.