Counter Intelligence

When viewing you settlements and noticing high unrest, could be a good chance you have a spy in town. Plop one or 2 down and flush them out. I ususally have 2 now on my frontlines and its ammusing watching them send spies to fail. If one gets thru I just try plopping a 3rd.

Also spies are by far the easiest to level up, just spy everyone and thing around you. Diplomats, merchants,priests, etc. Youusually get a trait increase doing so.

For my garrisons, usually just the free milita units unless bordering an enemy, I try to find a hub spot that is my medieval rapid response armies.