The old definition of a "city" (as opposed to a town or village) was a place large enough to have its own bishop. The bishop, in turn, had a cathedral, or a church that contained his seat of power, called a "cathedra" (meaning "chair" or "throne").
Find a Roman Catholic, Episcopal, or Eastern Orthodox cathedral and take a picture of your GPS and of the sign in front of the building. If you can get a picture of the bishop's chair, all the better.
Rules are: one log per diocese and one log per cacher. You must have a picture showing your GPS and the sign (or part of it) in the same frame. Also include any information about the cathedral you are able to find, such as when it was built, architectural features, the name of the current bishop, etc. Good luck!