Larger cities control larger areas. In this zone of control, your troops will automatically regenerate and supply cost will be lower than if they were in the field. Building an outpost in a strategic location accomplishes a similar goal. Zones of aggression surround your troops and those of the enemy, and when they cross combat take place. You may order your units to not engage and attempt to run by them without combat if you like.
Different postures allow for better defense, offense, or just plain speed of travel.
The game gets much more detailed than this, but you get the idea. Kohan comes complete with two training campaigns, and one large main game campaign. While this makes the game tightly scripted, it does allow the game designers to tell a story with the campaign, making it more interesting although it makes replay undesireable. Only one side of the conflict is available to play in single player mode, but this is the point of view for which the story is being told. The training missions give you a good idea of how the game is to be commanded, and how things work.