Originally Posted by Roethen
The only reasons I can imagine for a creature having advantage with ranged attacks on the high ground are:

1. If their body is partially covered

2. If their body is obscured by sunlight (but in order for this to ever happen the devs would need to implement the day/night cycle; something to consider).

You might want to do some research in military strategy if those are the only 2 you can come up with.