Brian Sullivan and Jeff Zettel look at a gravestone amongst the Phipps family in the Phipps Burial Ground, one of the stops on the new Charlestown Preservation Society’s walking tour app. The Phipps Burial Ground is one of the oldest in Boston and is typically closed to the public, but was open last Sunday, May 20, for the premiere of the walking tour app and a BBQ fundraiser for Memorial Hall renovations.