Static defense= militia and other things.
Simply put: noticing way too much raiding being way too effective, for both player and AI. FE is always going to have the stack of doom issue, so make the game revolve around them more.
Suggested fixes:
Allow cities and conclaves to build walls and other defensive structures. Limiting it to fortresses just makes no sense.
2nd change: unrest impacts militia strength. High unrest= less militia, low unrest= more militia. around 25% unrest= current militia.
Militia should have 2nd-tier armor/weapons, except for fortresses, where it should be 1st tier.
Outposts should also have a minimum strength needed to enter, so single units can't just spread havoc easily.
One other change, have a lower-tier siege unit, and give siege units a combat bonus in city battles.