The Zoo is a massive fortress-like facility, which sits in a moat and is heavily protected by the Zoo Crimes Division of the City's police department. It has apparently been voted the safest zoo in the country two years in a row.
When the Roboparents' antics lead to Zim having to create a zoo on his base's yard to maintain his cover, the ZCD investigate allegations of it being fraudulent, due to the fake animals on display. To try and avert their suspicions, Zim and GIR break into the real Zoo by dressing up as animals, in order to steal some real animals to populate his zoo with. This succeeds, the pair fighting off the ZCD's defenses and escaping, but shortly after tossing the animals away because their smell irritates him.