Leagues are the unit of distance on the realm map. Leagues are only seen when upgrading the watchtower, but are otherwise small compared to distances usually traversed by armies. Thus, leagues are not an indicator of distance in the game system, let alone used by players. Time needed to travel is a better way to represent distance.
When a player sends out an army to attack a target, it is more likely than not to have that army go around an obstacle on the realm map such as a lake or a forest. Terrain can add minuets of time in traveling especially over long distances. Other than time however, it doesn't affect the attack at all.
A player may start or move their city to a particular location where the terrain forces an incoming army to come to a "bottle neck" point in order to reach your city. With a decently upgraded watchtower, one could see incoming attacks coming through this point whole minuets before impact. This would help to allow last moment defenses should the defender be online and alert.