Research something like 'Underground Route' if your built resource is being attacked outside your city then your city stationed defense will defend it?

Also I think your current researched wall defense should apply for built resources inside that cities influence.

Underground Route was just the first thing that came to mind, it could be like Teleportation Network or something.. Point is when you have  5+ cities and each of them has 3+ or more resources it's a little to much micro to keep sending guards to 15 or more locations. Right now I try to build my cities so they connect to the resource but when once resource is at the end of my map and the other is at the oppisite side well.. My city looks funky

Just an idea if you can improve it please do.