No person shall keep, shelter, or harbor for any reason within the City a vicious animal except in the following circumstances:
1. Dogs used while in the line of duty by the police department, any other law enforcement agency or unit of the United States Military Service.
2. The keeping of guard dogs; however, guard dogs must be kept within a structure or fixed enclosure at all times, and any guard dog found at large may be processed as a vicious animal pursuant to the provisions of Section 57.04. Any premises guarded by a guard dog shall be prominently posted with a sign containing the wording “Guard Dog,” “Vicious Dog” or words of similar import, and the owner of such premises shall inform the Police Chief that a guard dog is on duty at said premises.