There is the concept of height. Units reveal the fog of war around them up to a certain distance unless they are a ground unit and the tile is higher than them

There is the concept of height. Units reveal the fog of war around them up to a certain distance unless they are a ground unit and the tile is higher than them. This leads to things like marching your units up a ramp, and not seeing that there's a ton of enemy units in there until you're right in the middle of them. Also, if units are walking through a valley, units on the high ground can shoot the ones on the low ground but not vice versa.