No person shall keep, shelter or harbor for any reason within the City a vicious animal as defined in this chapter, except in the following circumstances:
1. Animals under the control of a law enforcement or military agency.
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. 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.