Maybe you could add pillaging as a mechanic, but that supports raiding more than anything else.
Pillaging is a thing in Civ V. You break the improvement that's on a tile and get some gold for it.
I've got some peculiar conditions I'm pursuing here, which makes this doable but tricky.
The short version is that I don't like the notion that war is destructive, because that makes it a bad idea. Even when it's only a bad idea for whoever's not doing it, I'm still not fond of it, but especially when two players go to war and that means both of them lose by virtue of being at war, it seems like it makes the game worse.
So in essence, I don't want war to be a bad thing because then doing it is a dick move and/or a poor strategy. I'm perfectly fine, on paper, with war not having any negative consequences whatsoever...
...except that does really, really weird things lore-wise. Orcs raiding you routinely? Eh, we don't need any soldiers, they're fine. Rampaging dragon terrorizing the countryside? Probably just lost, it'll find its way out eventually. Worse still, this can become codified and mutual- you might have situations where a player intentionally settles near another to give them something to raid, or as a means of leveling their sole garrison unit.
So I do want war to be a
competition of some variety, I just don't want it to ruin anyone's fun or need a lot of success to pay for itself. This means, naturally, that pillaging should reward the victor with something and cost the defender something, but that it shouldn't be a big deal in any case. Given the mentioned lack of highly granular, semi-unimportant resources like currency, that's a tricky order. Maybe a stacking Happiness penalty (but again, was hoping to make Happiness a Thing You Do, not a condition) that's reset on winning a battle for the loser, and a Phallic trophy for the victor? That'd mean you couldn't
ignore orcish raids, but they'd be a temporary nuisance in most cases... though if they raid you a bunch and then
leave, you've either got a penalty you can't do anything about or the problem solves itself after all.
TL;DR Warning: Carebear Archlord designing war systems.
Other options are adding NPC villages, supply caravans and supply ouposts, which all would prevent decay, and perhaps military outpust that can heal.
NPC villages are intended as a large part of the game, but they'd only really be useful if you could convince one that's pretty close to the action to host you, meaning their neighbors haven't already recruited or coerced them for some reason. Maybe even nominal allies would be open to, you know, just... hosting some travelers from time to time, even if they happen to be riding wargs and dragging carts of war supplies and boasting about how those fools will never know what hit them?
Also, the defender still has a descive advantage. He starts using more troops, while the enemy already suffers from attrition.
Yeah, but his troops might be more spread out, he might not be nearly as warlike as the aggressor, and in theory he's risking something that the attacker isn't.
...phallic structures....>.>
Huehuehuehue.
I mean, call a duck a duck, right?
Gah. I don't really get how you win the game, anyway, since Irony said war isn't meant to be a way to conquer your enemies and see them driven before you.
Oh, that's easy. You "win" by having an empire you think is really cool. Despite all the mechanics, it's still intended as a sandbox more than a series of goals.