Corrected entry: The American Umbrella Corporation's main entrance is protected by just a mere wire fence and stands only by a few feet, yet for some unknown reason the numerous zombies are unable to climb over it. However, in the Las Vegas scene they obviously had no trouble climbing the much taller Eiffel tower which is a much more daunting task for something of limited motor capabilities.
Movieman123
2nd Oct 2007
Correction: No the "Bio-hazards" that are let go in the Las Vegas scene are zombies that have been enjected with a special anti-virus so they have way more intelligent motor function skills and reasonble reasoning. This is explained when The Queen tells Alice that the doctor has been bit by a specialy enfected "Bio-hazard."
Movieman123