Game difficulty difference
 

What do you exactly get as you go up in difficulty?

I read more enemies and higher level enemies, but I feel that I am not seeing that going up from low to average.

Also, do you have fewer turns to achieve Gold, Silver, or Bronze victory?

I know that some of the enemy units have different amounts of experience points depending on what level you play at.
It's not easy to see unless you open a campaign map with the editor.