No person owning or having charge, care, custody or control of any dog which has been trained to attack persons or animals shall cause, permit or allow the same to be or run at large upon any private property or premises including those of the person owning or having charge, care or custody of such dog except within such portion of such property or premises which are enclosed by a structure, fence or wall not less than five feet in height. Any gate providing direct access to such enclosed portion of property shall be capable of being locked except when in actual use. The fence, wall or structure shall conform to all applicable building requirements. (Prior code § 3-3.1)