It is simply, forget the type of troop, the rule apply to all (inf, cav, monster, etc..) and says:
Charge bonus +1 in open. While a hill is not open, there is no charge bonus if you end on the hill.
Another rule regarding the hills: Infantry & artillery are the only types of troop that can benefit of being in a defend or fortified position. (it is easy to understand that chariot, giants, cavalry and machines can't hide in defensive position).
So, the crest of the hill is sometimes intended to be a defensive position if the enemy charge from the downside the hill. As the enemy need to climb the defenders can use the terrain, rocks, slopes,etc as defensive. But only infantry and artillery can do this.
In example
So in the case an enemy start the charge in flat terrain and attack to an infantry in the top of the hill. The enemy (whatever type of troop do not benefit of +1 charge because the clash in the top of the hill not in open), and the infantry is defended (only can hit 5+).
This defend status is only for the first charge, sucesive pursuit does not apply (it is intended that after the first clash the defenders are out of theri hideouts, can't use the rocks and the slope, etc...)
Another case, an enemy start the carge in flat terrain and attack to a cavalry in the top of the hill. The enemy (whatever type of troop do not benefit of +1 charge because the clash is in the top of the hill, not in open), but can hit with 4+ because de defenders are a group of riders.
hope it helps
