Questions about the gameplay of Heroes 6.

Could you provide some tips on completing the campaign?
Does revealing the map affect enemy heroes attacking you?
While my hero is collecting resources from neutral creatures, a mercenary is exploring the territory, revealing undiscovered lands. And as soon as I reveal the bridge where an enemy hero is located, he starts moving, deliberately collecting resources and approaching my cities.
So, it seems that I shouldn't reveal new lands until I've collected everything in the already explored territory, right?