Why do you need garrisons, and how do you use them? In my opinion, garrisons are not very useful. If you leave a unit in a garrison,
it will be defeated quickly without a hero. You will only lose part of your army for no reason. I usually place garrisons on my maps only for aesthetic purposes.
Strong neutral armies are placed in garrisons so that the player can level up, explore everything, and then attack enemies. The area between the garrisons is filled with interesting things.