Fog of war keeps the tension high and makes this game full of surprises:
Every unit has a LOS (line of sight) range of 2 tiles and natural obstacles could hide many things ;).

Only Scout can see a unit hiding in forest from 2 tiles away (as you can see on the example here).
Red scout revealed the blue Archer but red Archer still cannot shoot at him because of the hills between them. He can shoot at the Horseman though since both his Pikeman and Scout revealed him.

Something I haven’t foreseen is players bluffing: moving hidden units only 2 tiles until the opponent comes closer only to reveal those are horses (they can advance 4 tiles in one turn)! That can be very dangerous if they catch you off guard