In general, there is a very large variation in the generated values... the same objects can be guarded by completely different army strengths, and this is not dependent on the type of your troops - sometimes it can be 2-3 times harder to break through.
http://dreamcatcher.metroland.ru/templates/units.html
and there is a direct formula for the number of guardians based on the importance of the object on the map. That is, if the map location is rich, then the guard on it will be stronger.
