Homm5 Skill School

I decided to play Heroes of Might and Magic V and am going through the campaigns. I have a question: is there any way to protect my units from enemy archers? In Heroes of Might and Magic IV, there was an option – ranged attacks and some spells required line of sight. Here, no matter how I position my units, enemy archers always manage to hit any unit. Is it impossible to protect against ranged attacks? The manuals say to use single units as cover, etc. Can you only protect against melee attacks?