That would be awesome, would love for wall upgrades to have much more meaning and drastically increase the time and men needed to take the settlement.I wish castles wall upgrades were significant, a citadel should be almost impenetrable with a decent garrison with less attack options for the sieging army. The Empire should have much better defensive settlements imo, maybe a capital like Constantinople with almost impossible walls to take by force.
Would be cool if the empire had a capital with a unique 4th level wall that made it nearly impossible to take, to control it should be the main goal of the 3 imperial factions split in the civil war and they could even make a special event like if an imperial faction controls the capital + 2/3 of imperial lands they get renamed Calradic Empire and get a new banner meaning it finally achieve legitimacy to claim the united imperial title.