The besiegers should lose more often than they win when besieging major, built up fortifications during the first couple of centuries.
For this to work, you would have to overhaul the fortification system substantially along with the siege system?
Not saying it would be a bad thing, but you would need a lot more granularity to fort construction and maintenance. I sort of think updating your forts and adding things like food stores, water sources and defences could be fun. Would then mean choice between a cheap but weak fort to briefly slow armies and heavily defended refuges that are very expensive to build and maintain in different areas. Something like different upgrades could reduce the risk of some of the negative rolls for defenders.
Would want a counterbalance for the attackers though...
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