The city wall is the incentive to leave your army to defend a direct attack against your city. It is effective. The fear of 40K Spearmen behind a city wall, even on plains, is enough to keep a 20K cavalry from wanting to attack. Throw in potential terrain and prestige bonuses and it gets worse.
Defending an attack is doable. The big problem is that there isn't that great of an incentive to attack a city. If I declare war on Sloter(sorry, using you as an example), why would I want to direct attack his city? At the most i would be sending my men to die in an attempt to steal basic resources. The only time I would want to do that is if Sloter has no allies and I can "blockade" him and cut him off from resources by constantly stealing his basic resources.
So stealing resources can be effective, but not if it cost me 4 months worth of military units and all the resources to make them. There isn't a very good trade off at all. Especially if you consider using crafted weapons and armor. hours and hours of gathering spent just to steal 200,000 iron?
I propose a different model. One that no one is going to like: Taxation. If I send an army to Sloter's town and kill all of his men in a bloody battle worthy of remembrance in the Jedi archives, I should be able to leave my army there and force his town to pay me taxes. 1/10 of all gold produced gets sent to me. 1/10 of all military created gets sent to me. 1/10 of all resources gathered gets sent to me. And lastly 1/10 of all resource crafted gets sent to me.
If Sloter doesn't like it, he can try to remove my troops from his town.
This Marshall Law/taxation would result in a new tactic to be used for war. New strategic targets (instead of plains cities with a neighboring large mountain) when attacking enemies. Large incentives to not let enemy troops into your city. Allow more strategy instead of stacking the largest army. Now a small army can be just as big of a nuisance as a large army.