Aspect which is often neglected and requires active behavior to be viable as income source. But after reading so far, you will come to realization that it is really lucrative. As you probably know razing single farmland gives you 50 coins. Not to shabby. But it also creates losses on whoever the owner of that hex was – he losses in short time 4-6 income from grain taxes and if it creates food shortage additional 6 income from Salt Tax and People Contribution which results in 10-12 coins per turn less. We aren’t even including aspect of slowing down or resetting settlement development.
Now let’s look at aggressor gains. You need unit to loot with and assuming you won’t do this with settlers you most likely will use either Militia/Tribesmen or Noble unit as they lack recruitment cost. If you lose them it won’t be as big of a deal. Nobles are more expensive but can deal better with limited attacks. After quick math you realize that single unit of tribesmen after reducing their upkeep creates income of 35 coins per turn of raiding farmlands. That’s equivalent of 8-9 farming population gold wise. So it really accelerates your economy. So you really should loot as much as possible to gain more of advantage.
But you must be also be smart when deciding on how much troops will you send towards neutral/enemy settlements. If you send too many (especially on GC) it will turn into loss as they will raze countryside to quickly and then will have nothing to raid. Your objective is to spend the minimum amount of time traveling and maximum amount of time looting. So spread your units across several settlements. Don’t divide them too much though as you will have to defend against sallying out defenders.
Looting settlements is different subject. Actually it is my preferred way of treatment towards newly contested cities. As it requires stronger military force and is harder you definitely want to make sure it will be profitable. City wealth depends on it’s size and structures inside it with Palaces/Schools&Temples and income/trade generating buildings being the richest targets.
After calculating cost to reward, you will want to target cities for gold in mid-late game rather than at the start. Capturing mid size city can grant you around 2000 coins and valuable bargaining chip for future peace negotiations. Money earned from conquest can be easily spent into your own huge construction projects. And after you’re done with city you can either keep it, leave it for enemy to retake (so you can loot it in the future), ransom it for money or use into persuading someone to become your vassal. Or anything else you want as settlements are really valuable on diplomacy screen. And you can be an a****ole if you want, by destroying all structures previously build by owner so he will have to spend money again :)