Havent had a whole lot of high end settlment sieges but citadels are pretty bugged
A regular citadel fires arrows, a citadel with ballista towers still fires arrows and a citadel with cannon towers fires ballistas at least that's what's happened so far in campaign and also attempts in custom battles.
As far as cities all has worked correctly for me at least so far in campaign and custom battles.
Large cities can have ballista towers, huge cities automatically get the ballista towers with the ballista towers being a prerequiste for cannon towers.
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