age of wonders 3 units that have evolved maintenance costs

(You can see this by hovering over their shield on the strategic map.

The campaign difficulty mostly changes the difficult level the individual AIs are set to. No defense, save a handful of cannon fodder to fight off the odd scout. Sam and Jacco are quite right on defenders.

Scout to make sure your armies are where you need them to be. Its not just symbolic, it actually hurts your income significantly. Indeed, the upkeep is to high to warrant that. TLDR: Never have a unit around that's not doing anything useful. (not as simple as it sounds, but it's a very important thing.) Just pay attention in tactical combat not to lose anything. For most other moments tho, a single stack will be just find if you can field a good one. If you have three strong stacks, you can explore three directions, then combine them only for a moment when you find enemy troop concentrations, or heavily guarded cities. If your main army isn't moving towards a new location on the map with the intent to do something useful, you're turtling. This will make it impossible/very hard for the enemy to take them out.

Park your scouts on mountains and water wherever possible. For example, you'll want to have like 2 stacks behind a wooden wall at the village where you find Reskar, for as long as your main army is out exploring to your west. Defenses are good if you want to focus somewhere else first.

So as long as your main army is camping on his doorstep, if you are strong enough to beat him in the field, it won't leave it's home. The AI won't attack when it feels pressure. Instead, you'll be using your scouts (you case, crows) to make sure you know to see the AI armies coming, so you can intercept them with your own army, probably your main army. Upkeep costs are just to high otherwise, and there's lots of things you want your cities to do. Generally, you'll want to see if you can do without any form of defense.

Age of wonders 3 units that have evolved maintenance costs